Gran killer thought drugs ruined his brain

An alleged murderer who stabbed his grandparents because he thought they were "vampires" later said his heavy drug use had ruined his brain, his trial has been told.

But an expert says it's common for people with psychotic symptoms to make illogical attempts to explain the emergence of psychosis.

Murray Deakin, 22, has pleaded not guilty on the grounds of mental illness to murdering his grandmother Gail Winner and retired police officer Mick Horne on the NSW Far South Coast in June 2018.

He was subsequently diagnosed with schizophrenia, but prosecutors say his psychosis on the afternoon of June 1 was caused by the ingestion of cannabis and lysergide (LSD).

His NSW Supreme Court trial heard that Deakin had apologised during a 2018 court appearance, said he had a family history of schizophrenia and that he thought "it was brought on by something".

"I've got a heavy use of psychedelic drugs," he said at the time.

He was also recorded telling his late father he'd "really f***ed my brain up with acid, I've gone too far with it".

"There are often illogical attempts to explain the emergence of psychosis," forensic psychiatrist Olav Nielssen told the trial on Tuesday.

"If you are a substance user, both patients and doctors can seize on that fact, overlooking the fact its symptoms have been there for some time."

After being arrested at gunpoint, Deakin muttered about the Illuminati and "a signal from the KGB subliminal messaging".

A doctor at Bega Hospital deemed him "clearly psychotic", noting a preoccupation with ideas of "thought injection, mind reading and what he called the matrix".

"At one point he spontaneously said 'I thought my grandparents were vampires so I stabbed them'," the doctor noted.

He later told police he believed Mr Horne was a demon and that he hit him on the head with a hammer "so it would stop him from being alive".

Dr Nielssen said Deakin "definitely" had prodromal schizophrenia symptoms in the months leading up to the fatal acts, including a decline in function and repetitive washing.

But another expert said it wasn't clear if Deakin's hallucinations leading up to June 2018 were signs of schizophrenia or symptoms of the drug use that commenced about 2015.

"Using drugs when you already have a predisposition to psychosis is like throwing petrol on a fire," Professor David Greenberg said.

Prof Greenberg was unable to say whether Deakin's schizophrenia was triggered by drug abuse or whether the acute psychotic episode on June 1 was drug-induced.

Deakin regained insight soon after the homicides, recognising the symptoms were not real.

"That improved over the next day or two, weeks and months," Prof Greenberg said.

"But he never fully recovered - he continued to have ongoing residual symptoms until when he was treated with Clozapine (12 months later)."

The trial is due to resume on Wednesday.