A 10-year-old boy found dead with head injuries following a house fire was unlawfully killed, a coroner has ruled.
However, Emma Carlyon recorded open verdicts on Ben Philpotts' mother, Patricia, and father, Harold, who died as a result of the same incident at Newquay in January 2010.
A badly burned Mr Philpotts, 47, was arrested 100 yards from the house and died in hospital but was the prime suspect in the fire which engulfed his estranged wife and son.
Ben, who used to live at Four Lanes, near Redruth, had been hit with a sledgehammer and was pronounced dead at the Royal Cornwall Hospital. His mother died at the scene.
At the three-day inquest in Truro, Dr Carlyon said evidence suggested Mr Philpotts had doused his wife's bedroom with accelerant, after petrol cans were found in his room, and a lighter in his trouser pocket.
But "there was no evidence as to how the fire was started".
Mr Philpotts, previously sectioned and with a history of mental illness, was convinced doctors and relatives were plotting to kill him. Experts told the inquest there had been no reason to believe he posed a risk to his family.
David Alexander, his social worker, said: "At no time did I have any concerns for their (Ben and Mrs Philpotts') safety because at no time did he say he wanted to harm them – rather the opposite."
Anand Irpati, from the Cornwall mental health team, agreed Mr Philpotts' condition did not present significant risk to the others.
He said: "It was considered he presented only a small risk to others. He had suicidal thoughts when distressed but ... denied having thoughts to kill himself."
PC Robert Hardwick told the inquest Mr Philpotts, when found, said he should have died in the fire himself and wanted to die.
According to his brother, Don Philpotts, Mrs Philpotts struggled to cope with her husband's mental illness while they were together "with little support from anybody".
Concerned family members initially persuaded Mr Philpotts to seek help from a doctor and then mental health services, but he later disengaged, believing he was physically, not mentally ill.
He was eventually sectioned and given anti-psychotic medication.
Towards the end of 2009, Mrs Philpotts became increasingly concerned about her husband's fluctuating mental health and worsening ranting and aggressive behaviour.
She sought a separation and was concerned over Ben's welfare.
Don Philpotts said: "Family members feel let down that both Pat, and by implication Ben, were not given enough help, support and guidance.
"Further, it is very concerning that knowing how irrational Harry had become that mental health services seemed to expect Pat to take all the burden, and they seemed to downplay the level of Harry's illness despite the many concerns expressed by Pat and others."
Cornwall Partnership NHS Trust said its investigations concluded "the incident could not have been predicted or prevented".
Ellen Wilkinson, medical director, said: "Mr Philpotts received treatment from the trust's mental health services.
"He found it difficult to accept his mental health and his engagement was neither consistent nor sustained.
"The trust made repeated efforts to provide him with treatment.
"Staff also offered support and advice to Mrs Philpotts."
She added that procedures had since been changed to improve patient care.