Ipswich Crown Court heard that Jason Mitchell, 24, had "auditory hallucinations" telling him to carry out the killing spree last December.
The court was told that Mitchell had a long history of mental disturbance and was committed, indefinitely, to a secure mental institution at the Old Bailey in 1990 after attacking a church cleaner with a baseball bat.
However, he was later released for care in the community, the court was told. Suffolk Health have announced a public inquiry to be led by Sir Louis Blom-Cooper, who was chairman of the Mental Health Act Commission.
Arthur and Shirley Wilson, both 65, were found strangled in their home in The Street, Bramford, Suffolk, on December 14.
Six days later, police broke into the home of Mitchell's father, Robert, 54, who lived just 200 yards away from the Wilson's home. They found Mitchell sitting in the dark. His father's torso was discovered in another room with his head, arms and legs packed into holdalls in the loft.
Mitchell, unemployed and of no fixed address, had denied murder but pleaded guilty to manslaughter on the grounds of diminished responsibility.
David Stokes QC, prosecuting, had told the court that after his detention in 1990, medical reports said Mitchell had had hallucinations telling him to kill.
Mr Stokes said: "He knew the Wilsons were a suitable couple. He said he had wanted to kill his father since he was six years old. He wanted to kill and eat someone, but the Wilsons and his father were too old to eat."
During his time on remand in Norwich prison, Mitchell exhibited bizarre behaviour and giggled constantly. Psychiatrist Dr Hadrian Ball said Mitchell had "clear features of mental illness, a psychotic behaviour".
He added: "He is schizophrenic. He is seriously mentally ill. He was ill while in custody and before that at the time of the killings. It distorted his judgment and responsibility for his acts. He's an extremely dangerous individual."
However, the court was told that another psychiatrist, Dr Raymond Goddard, described Mitchell as "a pleasant young man with no malice in him". He could find no underlying illness and believed his behaviour was caused by drink, drugs and a disturbed lifestyle.
Dr Ian Wilson, from Rampton Hospital, also assured the court that a medical review tribunal was unlikely to free Mitchell again.
But Judge John Blofeld QC rejected defence pleas for Mitchell to be committed to Rampton and said: "What I fear is that what happened once, will happen again." Imposing three life sentences, he said it would be up to the Home Secretary to decide on any possible release.
The case prompted Mind, a leading charity for the mentally ill, to appeal tonight not to regard the mentally sick as violent killers posing a danger to the community.
Mind's national director Judi Clements said: "The overwhelming majority of people diagnosed 'mentally ill' are not violent and pose no greater risk to the community than anyone else.
"Only a very small number of people who are a serious risk to others need to be in secure hospital provision when they are disturbed. Jason Mitchell had previously been known as a risk. Lessons need to be learned from this tragedy."
The inquiry into the case will begin in September and Suffolk Health hope it will have been completed by early next year.Reuse content