A woman killed by her mentally ill grandson had written a letter to medics "begging" for them to treat him two months earlier, a court has heard.
Conor Clarkson punched Marlene McCabe, 71, to the floor and pinned her down before inflicting a sustained assault at the family home in Blackpool.
He pleaded guilty at Preston Crown Court to her manslaughter.
Judge Robert Altham sentenced Clarkson, 26, to indefinite detention in a medium-secure hospital.
"I hope and expect that lessons are being learned from this tragic case," the judge said.
"It is difficult to imagine anything more his family could have done to try and get him treatment."
The court heard Mrs McCabe hand-delivered a letter to her local health trust outlining her concerns about Clarkson, which a court expert later described as an "almost textbook description" of onset and development of schizophrenia.
But in a phone triage assessment that followed days later in July last year, a mental health nurse ruled out immediate psychiatric intervention and Clarkson was not examined further.
'I am the devil'
On September 4, Mrs McCabe was punched to the kitchen floor by Clarkson and then pinned down as he inflicted a sustained assault using his fists and a weighted rope-knot doorstop.
The court heard she died from catastrophic head injuries.
When Clarkson's aunt returned to the house in West Park Drive, he chased her into the street and said: "I am the devil. That's what happens when you mess with vampires."
Clarkson, who was cared for by his grandmother since he was a child, admitted manslaughter on the grounds diminished responsibility at an earlier hearing.
Prosecutor Rob Hall said Clarkson started smoking skunk cannabis at the age of 13, withdrew from the world aged 20 and then used large quantities of cocaine two years ago.
'Light of her life'
Police were called in July when Mrs McCabe and her daughter Sharon Whitlow fled the house after threats from Clarkson who claimed to be Lucifer and a vampire.
He was assessed but discharged without a psychiatric examination.
Four days later the phone triage assessment took place with Mrs McCabe's letter acknowledged.
After the hearing, Ms Whitlow said: "Conor was my mother's only grandchild, he was the light of her life, she lived for him."
The family suspected Clarkson had schizophrenia but were informed he would be on a six-month waiting list before any assessment.
The family's lawyer Victoria Beel said: "We believe Mrs McCabe's death could have been prevented if Conor had received the mental health treatment she begged for."
Blackpool Teaching Hospitals NHS Trust has been contacted for a comment.