In the two-street town of Pennsburg, eastern Pennsylvania, stands a small house with an art collection you’d be unlikely to find in any public gallery. Arranged across the four walls of a downstairs room are pictures of injury and mutilation, of skulls in myriad configurations and colours, of women in positions of explicit intimacy, of folkloric landscapes and fantastical animals. But what sets these works apart is not so much their content as their origins: all of them were painted by serial killers.

This is the home of John Schwenk, who collects artworks and artefacts of murderers the way others might, say, rare stamps or film memorabilia. His most prized items include a portrait by John Wayne Gacy, known as the Killer Clown, a children’s party entertainer who raped and murdered at least 33 boys and young men in Chicago during the 1970s; a drawing of a skull by Richard Ramirez, aka the 'Night Stalker', responsible for numerous murders and sexual assaults in California in 1984 and 1985; and several pieces by Charles Manson, leader of the criminal cult the Manson Family, who orchestrated the brutal killing of the pregnant actress Sharon Tate and six others around Los Angeles in 1969.

As well as art, Schwenk owns thousands of letters from serial killers on death row, many of them addressed to him personally. They have sent him locks of hair, a prison shirt, a prison ID card, a set of false teeth, some unused dental floss and other oddities of dark provenance. He has got to know some of these pen pals and even considers a few of them friends. “I’m interested in what possesses somebody to kill another human being, and to do it numerous times,” he says. He acknowledges that one or two of them are “really scary”. These are the ones – the unreformed sexual deviants – who Schwenk’s wife Stacey says she hopes never get released, since they know where she and John live.

At one point Manson starts singing Don MacLean’s American Pie

I ask Schwenk to describe some of these friendships and what he finds interesting about them and he fetches a recording of one of his many phone conversations with Manson. It begins with Manson asking Schwenk in a slow raspy drawl where he was calling from. When Schwenk explains he’s at home in Pennsylvania, the celebrity criminal offers some observations about the Amish, after which he rambles from subject to subject in a virtually incoherent monologue that encompasses environmental activism, the Vietnam war, his pre-prison habit of sneaking into big houses (and, it should be said, killing their occupants), all the people who owed him money and what he was going to do to them, and “the new world order”. At one point he starts singing Don MacLean’s American Pie.

It is difficult to say what if anything this tells us about Manson’s mind (Schwenk says it was one of their more lucid exchanges). Yet you don’t have to share Schwenk’s preoccupation with the subject to find it fascinating. Interest in morbid crime, and particularly in serial killers, has become pervasive in popular culture. Jack the Ripper, the most notorious serial murderer of all – perhaps because he was never caught – has been immortalised, with considerable artistic licence, in hundreds of novels, comics, films and TV shows. Guided tours of his killing grounds in east London still attract huge crowds, particularly at night. Crime dramas such as True Detective, Dexter, The Fall and The Jinx draw audiences of millions. More than 70 million have downloaded 2014’s 12-part podcast Serial, which investigated the 1999 murder of 17-year-old Baltimore schoolgirl Hae Min Lee (previously no podcast had surpassed five million downloads).

There’s no sign of this fervour waning. Last October, a collection of 600 objects from the Metropolitan Police’s crime archives, which up to now has been off-limits to all but police officers, went on show at the Museum of London. Advance ticket sales were higher than for any of the museum’s previous paid-for exhibitions. In the world of populist criminology this is par for the course. In Washington DC, one of the most popular family attractions prior to its closure last September was the privately owned Crime Museum, where you could find curiosities such as Gacy’s 'Pogo the Clown' costumes and the oils he used to create paintings like the one on Schwenk’s wall; and the rusty, clay-coloured VW Beetle in which Ted Bundy assaulted and murdered dozens of young women in California in the 1970s.

Serial killers are responsible for less than 1% of murders in the US each year

Harold Schechter, an American true crime writer who specialises in serial killers, calls the popular interest in the subject “a kind of cultural hysteria”. Serial killers are responsible for less than 1% of murders in the US each year, and no more than two dozen are “active” at any given time, estimates Scott Bonn, a sociologist and criminologist at Drew University, Madison – yet our fascination far exceeds our concern about more pressing dangers. Why do we build this mythology around these troubled individuals? And what has that fascination taught us about their motivations?

‘Feeding frenzy’

The hysteria surrounding serial killers is nothing new. Serial killers and their ilk have attracted excessive attention since the rise of mass-circulation newspapers in the early 19th Century. William Corder, one of the last people to be publicly hanged in England after shooting his lover, was the subject of a “feeding frenzy” before and after his death in 1828, says Shane McCorristine, a cultural historian at the University of Cambridge. Corder murdered only once, and his crime was not particularly gruesome, yet he was as infamous in his time as Jack the Ripper later that century and Charles Manson today. His deeds were depicted in theatre productions, and in puppet shows at country fairs. Ballads about the crime sold as sheet music in their hundreds of thousands. Tens of thousands visited the barn in Sussex where the killing took place. At least 7,000 attended his execution. For several weeks that year, a piece of his scalp with ear attached was on display in a shop front in Oxford Street in London. “The extent to which Corder’s body was carved up, and his crime endlessly replayed, remains astounding to this day,” writes McCorristine in his book William Corder and the Red Barn Murder, published last August.

Astounding, perhaps, but not uncommon. In November 1957, police in Plainfield, Wisconsin, discovered the decapitated and disemboweled body of a local storeowner hanging by her heels in the kitchen of an isolated farmhouse. Elsewhere in the house they found several human skulls, some of them used as soup bowls; four chairs covered with strips of human skin; a belt decorated with female nipples; a blind-pull featuring a pair of female lips; a collection of female genitalia stored in a shoebox; another box containing four noses; a vest fashioned from the skin of a human torso; lampshades, a wastebasket and bracelets of similar provenance; and various other monstrosities, including nine human face-masks carefully peeled from the bone.

 Every weekend, families of sightseers headed to Plainfield for a peek at Ed Gein’s slaughterhouse

The owner, Ed Gein, eventually confessed to two killings and to digging up from local cemeteries the corpses of middle-aged women who resembled his dead mother. Gein became famous worldwide as the inspiration for the fictional Norman Bates in Alfred Hitchcock’s 1960 film Psycho, based on the book by Robert Bloch. He was also the model for Leatherface in The Texas Chainsaw Massacre and Buffalo Bill in The Silence of the Lambs. But in Wisconsin the gawping and the speculation – “extreme rubber-necking” as McCorristine calls it – began immediately after his arrest. Every weekend, families of sightseers from across the state headed to Plainfield for a peek at Ed Gein’s slaughterhouse. The property mysteriously burned down the following March, but that didn’t stop some 20,000 people turning up a few days later to inspect the place prior to it being auctioned.

Why do we find serial killers so enthralling? “They represent something larger than life, something truly cartoonishly monstrous, like the horror stories you’re told as a child,” suggests James Hoare, editor of Real Crime, a glossy monthly magazine that launched in the UK last August (its first two issues featured “the world’s deadliest serial killers” and Charles Manson). “Everybody responds to the idea that there’s something nasty out there.” Schechter calls tales of serial killers “fairytales for grownups. There’s something in our psyche where we have this need to tell stories about being pursued by monsters.”

The crimes of serial killers are frequently monstrous. Jeffrey Dahmer, the 'Milwaukee Cannibal', liked to boil and retain the heads of his victims and have sex with their corpses. Albert Fish, the 'Brooklyn Vampire', tortured and mutilated children before killing them. But arguably what really makes serial killers compelling is their humanness. An investigation into serial murder by the FBI’s Behavioral Analysis Unit in 2005 concluded that “they are not monsters and may not appear strange. Serial murderers often have families and homes, are gainfully employed, and appear to be normal members of the community.” Indeed police considered Dahmer so unthreatening that they unwittingly returned one of his victims to his care; Fish’s neighbours considered him a kindly, child-friendly old man; and Gacy, in addition to being welcomed as a clown at children’s parties, won recognition for his fundraising work.

They’re charming, almost unbelievably so, charismatic like a Cary Grant or a George Clooney – Helen Morrison, forensic psychiatrist

Who are these people? Are they something alien, or one of us? With no agreed pathological profile, it is hard to say. Helen Morrison, a forensic psychiatrist who has interviewed more than 80 serial killers and was a defence witness at Gacy’s trial, has found them to be expert role-players, adept at appearing normal. In her memoire My Life Among the Serial Killers (Wiley, 2004), she writes: “I never quite know whom I’m dealing with. They are so friendly and so kind and very solicitous at the beginning of our work together… They’re charming, almost unbelievably so, charismatic like a Cary Grant or a George Clooney.” Bonn, the sociologist, thinks this is part of their appeal, and also what makes them terrifying. “Look at a guy like Ted Bundy. He was very good-looking, he was successful, women were very attracted to him, which was why he was able to get 36 of them into his car [before abducting and killing them]. He looked like the boy next door, and that is frightening because if the boy next door is a serial killer, it means anyone is potentially a victim.” Particularly since their victims are almost always strangers (though this is not true for female serial killers, who tend to kill people they know).

 In September 1978, the serial murderer Rodney Alcala took part in an American TV show called The Dating Game

For an illustration of how easily serial killers blend in – and why police investigators rarely catch them early – take the case of serial rapist and murderer Rodney Alcala. In September 1978, he took part in The Dating Game, an American TV show in which a single woman – in this case drama teacher Cheryl Bradshaw – got to question three single men hidden from her view before selecting one based on their answers. Unbeknown to anyone, Alcala by this point had raped and killed at least two women in California and two in New York. On the show he appears witty and charming, with coiffured hair and a flamboyant shirt and suit. Bradshaw picked him. But after talking to him backstage she decided not to date him, thinking him “creepy”, a judgement that may have saved her life. Over the next two years, Alcala raped and murdered three more.

The apparent normality of serial killers – the juxtaposition of horror and humanity – fascinates enthusiasts like Schwenk, whose letter-writing and collecting is partly an attempt to understand what makes them tick. “They look like everyday people, they act like everyday people. Like you and me. Many of them are nice, regular guys. There’s just something upstairs that isn’t right,” he says.

Steven Scouller, a documentary-maker and collector living in Scotland, has gone one better: he’s befriended a killer who has been released from jail. Nico Claux, who served eight years of a 12-year sentence, is not a serial killer, having being convicted of only one murder, but he was a serial consumer of the dead, stealing body parts from graveyards in Paris, eating the flesh of corpses at a morgue and drinking blood from a hospital blood bank after taking it home, cooling it in his fridge and mixing it with human ashes.

Scouller says he and Claux are the same age and like the same music and films. “We have a lot in common. He’s an interesting guy, he’s funny, polite, well-mannered. He’s full of remorse, he’s not happy with what he’s done.” Recently Claux took Scouller to the Père Lachaise cemetery in Paris - final resting place of Oscar Wilde, Jim Morrison and Chopin - where he did much of his grave-robbing. “You can read all the books in the world about serial killer cannibalism, but until you’re face-to-face with somebody telling you this is what I did, this is where I did it… it’s a privileged situation to be in, albeit macabre.” Afterwards, Scouller recalls, they went out to dinner, and Claux ordered steak, and he asked for it rare. (You can view some of Scouller's collection in the gallery below.)

One late-summer afternoon I visit the painter Joe Coleman in his apartment, high up in a red-brick mansion block in New York’s Brooklyn Heights. Coleman’s art is vivid, intricately detailed and frequently apocalyptic, akin to religious iconography. It is in high demand, sought by the likes of Iggy Pop, Johnny Depp and Leonardo DiCaprio. Coleman is also known for his interest in the dark side of human nature, and for personifying it. He opens the door dressed in a black three-piece suit and black tie, an arrangement of occult keepsakes pinned to his waistcoat – a curved tooth, a miniature skull. His living room is a shrine of curiosities, full of ephemera of the sacred and profane: a mummified child, a two-headed antelope, shrunken heads, the death masks of executed killers, a deformed baby preserved in a jar, life-size waxworks of gangsters and murderers.

We sit on a couch opposite a prone effigy of St Agnes, a Christian martyr from the third century, which supposedly contains some of her skeletal remains. On the wall behind us is one of Coleman’s own paintings (of Mary Bell, who in 1968 as an 11-year-old strangled to death two small boys in Newcastle). It hangs beside one of his favourite artefacts, a calling card of the British executioner William Marwood, who invented the humane “long drop” method of hanging, by which he dispatched more than 170 prisoners in the late 19th Century. Elsewhere there is a bullet from the gun that killed Lee Harvey Oswald, a lock of Manson’s hair, an undershirt worn in the electric chair by murderer Elmo Patrick Sonnier (played by Sean Penn in Dead Man Walking), and artworks and letters from Gacy, Manson and others. He points to a frame containing perhaps the most iconic letter of all in this genre, written by Albert Fish to the mother of Grace Budd, his final victim, in which he describes how he strangled the child, cut her up, cooked her and ate her.

There’s a part of them that’s in all of us, and there’s a part of all of us that’s in them – Joe Coleman

Why are they here, these relics of the macabre? The historian McCorristine thinks that getting close to criminals and perpetrators of horror is a way of experiencing death without falling victim to it, of becoming a witness to death and thus exerting some control over it. Coleman says this is true for him, and that owning a piece of someone – a lock of hair or letter or artwork – reminds you of the dark forces that may lead someone astray. “I always felt there was a part of me that was really dark. When I was really young I tried to set the school field on fire. I did some terrible things, and I feel that there but for the grace of god go I.” He feels compelled to empathise with the protagonists, to acknowledge their humanity alongside their malevolence. He wants to recognise that “there’s a part of them that’s in all of us, and there’s a part of all of us that’s in them. If we can’t find some compassion or empathy for the very worst of humanity then what hope have any of us.”

As he is finishing this sentence, a cockroach close to two inches long emerges from beneath the effigy of St Agnes and scuttles across the floor towards us, disappears under the couch, then re-emerges on the wall behind, heading for his painting of Mary Bell. It contemplates a bleached death mask in its path and begins to traverse it, and Coleman can’t resist reaching for his camera. In most apartments in Brooklyn Heights such cockiness would put a cockroach in mortal danger, but in Joe Coleman’s museum of oddities it becomes just another exhibit.

The reverence Coleman feels for his objects, his sense of their power, is easy to understand if you have visited the Surgeon’s Hall Museums in Edinburgh and stood before the death mask of 19th Century serial murderer William Burke and the pocketbook made from his skin. Or if you have been to Washington DC’s Newseum and peered into the smoke and sweat-stained cabin where Ted Kaczynski, better known as the Unabomber, lived and plotted his 17-year campaign of terror. Each of these has consistently been the most popular attraction at its respective museum, almost certainly because of their association with serial murder.

Coleman has his lock of Manson’s hair; Scouller, a grey tuft from New York serial killer Arthur Shawcross – “I own a piece of him,” he says

In experimental studies, the psychologist Paul Bloom and his team at Yale University have found that we are all prone to magical thinking when confronted with such objects, believing that someone’s qualities or essence can pass into their possessions through physical contact. The objects become contagious. This applies most to actual physical remnants such as skin, hair or fingernails. In Victorian times it was traditional to keep hair from a deceased loved one (I have some from my great-great-grandmother). Such tokens are highly sought-after by serial killer afficionados, though proving the object’s provenance can be tricky. Schwenk has some hair from Dorothea Puente, famous for preying on the elderly occupants of her Sacramento boarding house; Coleman has his lock of Manson’s hair; Scouller, a grey tuft from New York serial killer Arthur Shawcross – “I own a piece of him,” he says.

Many people would be repelled by the thought of even touching such objects, but if you’re fascinated by serial killers, owning their body parts is almost as enthralling as having them sitting in your front room. That might sound a tad unhealthy, yet Schwenk, Coleman and Scouller never seem to question whether their obsessions are anything but normal. “It’s a hobby,” says Schwenk. “There’s nothing wrong with me.” For Coleman, his collecting is even cathartic, a way of “releasing the demons in a way that’s positive rather than destructive”.

Inanimate objects can be just as powerful as biological ones, particularly letters, which can reveal much about their authors (Manson’s are generally as incoherent as his conversations and his drawings), and works of art. Stephen Giannangelo, who lectures in criminology at the University of Illinois, acquired some of Gacy’s paintings to use as teaching tools and says they never fail to have a dramatic effect on his students. “When I bring them in, three-quarters of the kids who never look up from their phones are suddenly taking pictures of them and walking up to the front of the room and asking every question they can think of.”

Murderabilia

It’s not hard to imagine his students’ reaction were he to bring in something more ghoulish, for example the razor that Ed Gein used to fashion his human accoutrements, which is currently owned by Scouller. When I visited Scouller he let me hold it – it’s small and worn with jagged edges – but he wouldn’t tell me how he came by it. The trade in murder memorabilia – “murderabilia” as it is sometimes known – is opaque. Scouller obtained his first piece – his Arthur Shawcross hair – on eBay, but in 2001 the site banned the sale of crime-related memorabilia out of respect for the victims.

Several specialist auction sites cater for true crime collectors

This led to the flourishing of several specialist auction sites, such as Murder Auction, Serial Killers Ink and Supernaught, that cater specifically for true crime collectors. On these you can buy such desirables as a handful of soil from Gein’s gravesite ($25), the seatbelt from Shawcross’s car ($800), an unfinished burrito chewed by Manson in his prison visiting room ($800), Ramirez’s prison-owned TV ($4,200) or two of Gacy’s Pogo the Clown paintings with signed letters ($125,000). Eric Holler, who runs Serial Killers Ink from his home in Jacksonville, Florida, says objects related to famous serial killers can sell in hours, and that all kinds of people buy from him. “I sell all around the world, to men and women, the military, law enforcement, psychologists, professors of criminal law, or just your average collector.”

If Texas senator John Cornyn had his way, people wouldn’t be allowed to make money from murder. Since 2007, Cornyn has been trying to persuade Congress to consider a bill banning the sale of crime-related materials, so far without success. He and others believe the trade glorifies violence, rewards the killers (even though in most places they are not allowed to profit from their crimes) and pains the victims. The controversy didn’t dissuade the US government in 2011 from auctioning off the Unabomber’s personal effects, albeit to raise money for his victims. The sale included his manifesto against the “industrial-technological system”, published by The Washington Post and the New York Times in 1995 while he was still on the run; the typewriter he used to type it; the hoodie and sunglasses immortalised in his police wanted poster; and bows and arrows with which he hunted animals near his Montana cabin. Predictably, there was no shortage of bidders: the auction raised $232,246.

Many of the people who are drawn to the artefacts of serial killers are also drawn to the places where they killed. After murderers are captured, their homes and the scenes of their crimes often become pilgrimage sites. As the cultural historian Alexandra Warwick puts it, a murder site is seen as representing “a map of the interior of the killer’s mind. It is effectively his mind laid out, his work displayed and signed, a text to be read.”

The American photographer Stephen Chalmers recently turned this idea on its head in Unmarked, a project about the places serial killers in the American West dumped their victims, which he traced through public records and police reports. Most of the sites he chose are in wilderness areas close to hiking trails, and his photographs re-frame them as scenes of epic beauty. They are memorials to the victims, not the crimes. Chalmers had the idea after hiking near Tiger Mountain outside Seattle with a woman he was dating. “The sun was shining, the birds were chirping, we had a picnic, it was awesome.” Then one of their friends told them they’d been hiking in the area Ted Bundy disposed of his victims’ bodies. “Suddenly it turned from this beautiful courtship to something rather macabre.”

 

In each of the photographs in Unmarked his camera focuses on the precise spot the victim was found, and as viewers we are caught between the exquisite landscape and the trauma inscribed therein. The knowledge of what happened changes everything. Recently, Chalmers returned to the sites to collect flowers and grasses, which he has been drying and pressing in the basement of his home outside Youngstown, Ohio. He plans to include the cuttings with a limited-edition book of Unmarked that will be published this year, to strengthen the sense of connection to the places he has photographed. They are almost certain to become collectors’ items, and not just among the art-loving audiences he is hoping to reach.

Catharsis

One of the more provocative explanations for the appeal of serial killers is that they serve some kind of social function, allowing us to indulge our most vengeful fantasies without having to act them out, and, once the killer is caught, without having to feel guilty about it. “They’re almost like a catharsis for the worst of us, a lightning rod for our darkest thoughts, like the sin-eaters in medieval times who would take away the sins of others and by so doing cleanse society,” says Bonn. They also give us the opportunity to suffer death from a distance, to get “as close to the abyss as you can while not falling in”, as McCorristine puts it. This, he says, is why some people are compelled to watch Isis execution videos, even though they may later regret it. It could also explain why we slow our cars in the aftermath of a traffic accident, gawking for a glimpse of horror on the other side of the barrier.

Perhaps what we like most of all is to be terrified. I stand guilty. In 1995, I dated a girl in Paris who was convinced she was being stalked by a serial killer. The police seemed worried too. They thought her stalker could be the same man who had raped and stabbed to death four young women in her part of Paris over the previous 18 months. The police gave her an emergency phone which she could call any time, and a friend gave her a gun which she kept under her bed. She was terrified all the time. Frequently she refused to let me in fearing it was her stalker at the door. It terrified me too. But I was also transfixed, and addicted. Not that I told her.

Eventually the threat subsided. Three years later, the police arrested a man who confessed to the four mid-90s murders, and to three others. His name is Guy Georges, now known as the 'Beast of the Bastille'. He is serving life imprisonment with little possibility of parole. I have no desire to write to him, nor to solicit him for a painting or lock of hair. But I will probably watch L’Affair SK1, a film based on his story that came out this year. He will certainly be hard to forget.

 ---

 Join 500,000+ Future fans by liking us on Facebook, or follow us on Twitter, Google+, LinkedIn and Instagram.

If you liked this story, sign up for the weekly bbc.com features newsletter, called “If You Only Read 6 Things This Week”. A handpicked selection of stories from BBC Future, Earth, Culture, Capital, Travel and Autos, delivered to your inbox every Friday.