Myles William Taylor, 36, faced a judge-alone trial in the West Australian Supreme Court charged with murdering his parents at their Hocking home in April 2020.
He admitted intending to kill his mother Lesley Ann Taylor, 64, and 65-year-old father Michael Wayne Taylor when he repeatedly struck them in the head with a hammer.
But he pleaded not guilty to their murder because he was of unsound mind when he carried out the attack.
Justice Anthony Derrick on Wednesday acquitted Taylor of the murders, saying he was satisfied the 36-year-old had been suffering from paranoid schizophrenia and believed killing his parents was the only way to stop the voices in his head.
Taylor will remain in custody until his release is recommended to, and approved by, the state governor.
The judge was told Taylor had first experienced hearing voices as a teenager, and the condition had re-emerged in the 12 months prior to the killings.
He believed his parents were trying to "get people to kill themselves" and that people around him were able to read his thoughts.
On the day of the killings, Taylor threw a brick through a rear window of a nearby house then began striking his bedroom wall with a geologist's hammer.
Taylor then walked to the lounge room of the family home and hit his father with the hammer, knocking him to the ground, before repeatedly striking his mother in the back of her head.
He then took a shower and returned to the lounge room to find his father breathing and sitting upright, prompting him to strike the victim several more times, the judge heard.
He called triple-zero and told the operator to send a police car because he had just killed his parents, adding "I think everyone knows what happened".
His mother was pronounced dead at the scene, while his father died two days later in hospital.
A forensic psychiatrist found it was likely Taylor had been suffering from undiagnosed schizophrenia since he was a teenager and his drug use had likely worsened his psychosis.
Upon receiving treatment in custody, Taylor reported no longer experiencing psychotic symptoms.
"He was remorseful and felt extreme sadness for what he was alleged to have done," Justice Derrick said.
The court heard Taylor had called police seeking a welfare check about a fortnight before the killings.
When officers arrived at the home, Taylor said he could see faces in the bed sheets and birds on the walls but declined to be taken to hospital.
The officers told Taylor's parents they could not section their son but advised them to seek a restraining order if they feared for their safety.
Taylor made full admissions upon his arrest and disclosed having taken methamphetamine two days prior to the killings.
However Justice Derrick found Taylor, who was described as an honest and generally reliable witness, had not been intoxicated when he killed his parents based on the elapsed time and the amount of meth found in his blood.
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