There are many reasons why people might want to kill a tiger, not the least of which is self-defence, but in Bangladesh the killings have a surprising motive

Even today, people still kill tigers and other endangered species. One of the most important questions to ask is why they do it, because it is only by understanding people's motives that we might change their behaviour.

Globally, tigers are an endangered species. Bangladesh was once a stronghold for them, but today it is home to barely 100. Many of the survivors cling on in the country's south, in the vast mangrove forests known as the Sundarbans.

During the British colonial period, hunting dramatically reduced the Sundarbans tiger population. Hunting was outlawed in 1974, but since then poaching has taken a severe toll on the Sundarbans tigers.

So why do people do it?

A study published in the journal Oryx in October 2016 by Samia Saif of the University of Kent in Canterbury, UK and her colleagues suggests that there are four main motives behind tiger poaching in the Sundarbans.

The Sundarbans is plagued by organised gangs, known locally as "pirates"

The first reason is safety. Saif says there are a lot of man-eating tigers in the area, thanks to "a combination of deer poaching, human presence in the forest and a high tiger density". As many as 30 people are killed by tigers each year, so people kill tigers for protection. 

The tigers also sometimes eat a beef dinner, rather than their more natural diet of deer. This leads to angry farmers poaching tigers after having their valuable cows stolen from them.

The illegal wildlife trade is also a factor. There is strong demand for tiger parts in traditional medicine, and for skins for use as home décor, so the tigers are targeted by professional poachers keen to earn a quick buck.

The fourth and last reason why tigers are killed is more sinister.

The Sundarbans is plagued by organised gangs, known locally as "pirates". These criminals act a lot like the Italian mafia. They patrol their designated stretch of swamp, extorting and kidnapping fishermen, and generally terrorising the community. They are also known tiger killers.

A local villager, who wished to remain anonymous, spoke to Saif about these organised gangs.

"You can't trust the pirates," the informant said. "It is normal for them to kill tigers as they have illegal weapons and they kill tigers if they see them."

The pirate told me, 'see, we killed a tiger, so how long will it take to kill you!'

The villager recalled a conversation he had had with a local fisherman who had been kidnapped by the pirates, and who said that during his captivity he saw the pirates kill a tiger.

An owner of a fishing vessel, who has had 17 of his men kidnapped by pirates over the years, said: "My fishermen saw three tiger skulls in the trawler when they were kidnapped two months ago''.

One unfortunate villager was kidnapped three times by the pirates. "Last time they took me in their trawler, a tiger limb hung out from a sack," he said. "The pirate told me, 'see, we killed a tiger, so how long will it take to kill you!'"

A former pirate told Saif that, when he entered a new area of the forest, he would look for tiger tracks in the mud. If he found any, he would try to track the tiger down and shoot it. 

It is not clear how many tigers are killed by pirates. However, many people knew of pirates that had been involved in tiger poaching, and most victims of kidnapping witnessed large-scale poaching. What's more, the killing of tigers has become part of the local culture.

For instance, Saif found that pirates sometimes kill tigers as a gesture of goodwill towards the local community.

The pirates often butter up their victims using acts of "kindness", such as protecting them from supposed man-eating tigers and giving money to build mosques. It is rather similar to the way drug lord Pablo Escobar, whose life is dramatised in Narcos, handed out cash to the residents of his Colombian hometown.

The pirates are not scared of the guards because they are armed with modern weapons

What's more, in Bangladesh, just like elsewhere in Asia, tiger parts are used for "medicinal" and "spiritual" purposes.

For instance, some of the locals spoke about the use of canine teeth from tigers. According to one person who was friendly with a pirate, "The gang leader has a tiger canine locket. Normally the pirate wears the ring or locket when he comes to villages to meet up with their families, as it enhances sexual virility."

Similarly, a woman who was married to a pirate confessed that her husband gave her a tiger canine ring when she complained of joint pain.

This means it will be tricky to limit the tiger poaching.

There are guards within the forest who look out for criminals, but the pirates are not scared of the guards because they are armed with modern weapons. In 2009, pirates killed two forest guards.

The pirates expressed their interest to surrender to come back to the normal life

The Bangladesh Tiger Action Plan, published in 2009, recommends more boots on the ground to combat poaching. "The establishment of a specialist Wildlife Crime Unit would strengthen enforcement by creating improved capacity to investigate domestic crime and illegal international trade," it says.

In other areas of the world, poachers have successfully been turned into wildlife trackers. Saif says that this could also work in the Sundarbans, as the pirates know the forest well.

But there is also a simpler solution. Saif found that some pirates were ready to give up thug life. "The pirates expressed their interest to surrender to come back to the normal life," she says. "Recently, some pirates surrendered to the government after getting assurance that the government will try to minimise their punishment."

Join over six million BBC Earth fans by liking us on Facebook, or follow us on Twitter and Instagram.

If you liked this story, sign up for the weekly 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.