A 43-year-old man has been found not guilty of murdering his father in Perth's south last year on the grounds that he was of unsound mind.

Key points: Nigel Paul Chaytor stabbed his father to death in their family home, then turned the knife on his mother

Nigel Paul Chaytor stabbed his father to death in their family home, then turned the knife on his mother During the trial the court heard his father sought help after Chaytor reported talking to "spirits" and stopped eating

During the trial the court heard his father sought help after Chaytor reported talking to "spirits" and stopped eating He has been found not guilty on both counts, but remains in custody

Nigel Paul Chaytor had been charged with murdering his 72-year-old father Keith by stabbing him at the family home in Kelmscott in April last year.

He had also been charged with attempting to murder his mother Estelle.

In Perth's Supreme Court, Justice Lindy Jenkins found Chaytor not guilty on both counts due to unsoundness of mind.

She offered her sympathy to the family and told the court she hoped community health services would review the case.

She said Nigel Chaytor was left unmedicated and without adequate supervision in the community.

Keith Chaytor was fatally stabbed at his Kelmscott home and his wife critically injured. ( ABC News: James Carmody )

'Ancestors' told Chaytor to attack 'cult leader' parents

During Chaytor's trial, the court was told he was first admitted to hospital due to mental illness in 2014 and diagnosed with a delusional disorder and schizophrenia.

In the week before the attack, his parents noticed his mental health was deteriorating.

A mental health triage team noted on April 25 that he was not eating, believed he had a sixth sense and was talking to "spirits".

The team said someone would be sent to speak to Chaytor at his workplace but in the early hours of April 27 he carried out the attack at his parents' home.

Following the attack, Chaytor told police "it just needed to be done".

"I was told by a spirit it had to be done … to take them out by any means," Mr Chaytor said.

"I had to put them to sleep.

"My ancestors told me to do it. The doctors gave me tablets but I threw them out because I don't need them."

A forensic psychiatrist said Nigel Chaytor was unable to control his actions or know they were wrong. ( Supplied: Facebook )

Chaytor also told police his parents were evil and manipulative and claimed he had a psychic gift and spoke to people around the world psychically.

In a video-recorded interview with police played to the court, Chaytor maintained he had no illness and said he had stabbed his parents because they were cult leaders and he had to get rid of them.

"I've been tapped by the system, the tapping of a mind program going on around the planet," he said in the interview.

"I had to put two cult leaders to sleep under the psychic law."

Chaytor not taking medication for two months before attack

Forensic psychiatrist Victoria Pascu testified in court it was her opinion Chaytor was deprived of the capacity to control his actions and know that they were wrong.

Dr Pascu said Chaytor was hearing a male voice that was "commanding" him to take out the cult leaders.

"He determined he had to do the acts … he was told by the voice … he had to obey the command of this voice," she said.

Dr Pascu said Chaytor also believed his life would somehow be under threat if he did not stab his parents, saying he recounted a feeling of calmness afterwards.

But she testified he had described holding the knife by the blade to try to soften the impact of it, an act which delivered cuts to his hand that required surgery.

"Because he cared for his family he was trying to soften the blows to his father and his mother," Dr Pascu said.

A nearby park formed part of the area police searched. ( ABC News: James Carmody )

The court heard Chaytor's medical history suggested he did not have insight into his mental illness and he had not always been compliant with his medication and had questioned the need for it.

Dr Pascu said he told her he had not been taking his medication in the two months before he attacked his parents.

Chaytor has been held at a secure psychiatric facility since his arrest on the day of the stabbings.

He will remain in custody.