Alexander Lewis-Ranwell was gripped by delusions about saving young girls from a paedophile ring, Exeter Crown Court heard.
The 28-year-old battered Anthony Payne to death with a hammer at the 80-year-old’s home in the St David’s area of Exeter on 10 February.
He later scaled the wall of 84-year-old twins Dick and Roger Carter, who lived a mile and a half away in Cowick Lane, before bludgeoning them to death with a shovel.
Before returning its verdict, the jury gave a note to the judge raising concerns about the “state of psychiatric services in the county of Devon and the failings in care in Alexander Lewis-Ranwell’s case”.
Lewis-Ranwell had been arrested twice in the days before the “whirlwind of destruction” that led to the pensioners’ deaths, the court heard.
Just hours before killing Mr Payne with a hammer in an upstairs bedroom of his terrace home, Lewis-Ranwell had been released from police custody after attacking farmer John Ellis, 82, with a saw.
This was his second arrest in the space of 24 hours and occurred just seven hours after he had been held for attempted burglary at another farm.
His mother Jill Lewis-Ranwell had phoned police expressing “grave concerns should he be released” after the first arrest, the court heard.
Members of the public had also reported Lewis-Ranwell’s worrying behaviour to the police in the hours around the killings.
Shop staff had noticed him “rambling about random things”, while a taxi driver who gave him a lift told how he feared for his life because of Lewis-Ranwell’s “irrational” behaviour.
He was released from custody at Barnstaple police station at 2.49am on 9 February but returned there seven hours later after attacking Mr Ellis.
A 12-minute call with a mental health practitioner at 3pm identified “potential psychotic symptoms present including paranoid beliefs”.
An inspector reviewing his detention wrote at 4.11pm that Lewis-Ranwell “potentially presents as a serious risk to the public if released”.
A forensic medical examiner, a doctor employed by G4S Health Services, was escorted to Lewis-Ranwell’s cell at 6.30pm but deemed he was not “acutely unwell” and a full mental health assessment was not carried out.
Dr Mihal Pichui told jurors he left the police station with the “expectation” he would be seen by a mental health nurse the following morning but later found out this did not happen.
Lewis-Ranwell was released from Barnstaple police station at 9.32am and travelled to Exeter before killing Mr Payne and the Carter twins.
He was arrested for a third time a day after the killings when he attacked night manager Stasys Belevicius at an Exeter hotel.
Concerns were raised about his mental health while he was in custody and he was transferred to a psychiatric unit for assessment.
He later told a psychiatrist at Broadmoor secure hospital: “I cannot believe no one helped me – they let me out twice when I was unwell.”
Lewis-Ranwell also thought the police had “sanctioned his actions” because they had twice released him from custody.
The former scaffolder had been suffering from paranoid schizophrenia and gripped by false beliefs about saving young girls from a paedophile ring, the court heard.
One doctor said the defendant was living in a “very nightmarish world” and believed he had a “moral justification” for the killings because he was rescuing people.
Another psychiatrist said he was on a “quest” to rescue girls from a locked cellar.
Three psychiatrists agreed Lewis-Ranwell was insane at the time he killed Mr Payne and the Carter twins, but the prosecution argued the defendant bore some responsibility for what happened.
Lewis-Ranwell was issued a hospital restriction order after the verdict was read out.
Detective Superintendent Mike West, from Devon and Cornwall Police, said: “During his time in custody, prior to the deaths, the defendant had a number of interactions with five different health care professionals who were involved in providing guidance and professional assessment.
“As a result it was agreed that he was fit to be detained and interviewed and indeed confirmed that he did not need a full mental health act assessment.
“We fully accept our responsibilities to look after those detained in our custody units, however it is unreasonable to suggest that police officers or staff, in these circumstances, should have over-ridden decisions made by those who are trained, qualified and skilled in health care.”
Lewis-Ranwell, from Croyde, north Devon, had pleaded not guilty to murder by reason of insanity.