An ACT Supreme Court murder trial on Tuesday heard Christopher David Navin, 29, thought Mr Sofer-Schreiber had organised his grandfather's death in 2013.

"It was me or him," Navin told forensic psychiatrist Stephen Allnutt.

"[The] only way to stop him was to kill him."

Navin has pleaded not guilty by way of mental impairment to the Boxing Day 2013 slaying of Mr Sofer-Schreiber.

Mr Sofer-Schreiber was found dead on the floor of his Lyneham home on December 28, two days after he bled to death from 73 stab wounds.

The Crown alleges a soured friendship had been Navin's motive for the killing.

Dr Allnutt, who examined Navin and prepared a report for the trial, said the accused described how he had believed he had been watched by "observers" since 2011.

Navin told Dr Allnutt he had hid in the bush, travelled to Victoria and Tasmania, and eventually ended up in a north coast psychiatric unit as a result.

But Navin reported that the symptoms continued upon his discharge and he travelled to the United States after receiving messages.

Dr Allnutt said Navin claimed he began to hear voices and thought his mind could be read on the flight home.

Navin's grandfather died in early 2013, and he attributed the death to the work of the observers, who worked with Mr Sofer-Schreiber, the court heard.

In late 2013, he believed the observers wanted him to befriend Mr Sofer-Schreiber or another relative would be killed.

The court heard a piece of wire that looked like a noose convinced Navin that Mr Sofer-Schreiber intended to hire a hitman to kill his family.

He told Dr Allnutt that he had stabbed Mr Sofer-Schreiber with a paring knife until it broke, and then continued the attack with a larger knife he got from the deceased's kitchen.

The court heard Navin then took a tent and camp chair from the Lyneham unit as he believed it had been a sign he should go to a Grafton property.

Earlier in the day, the jury heard Navin would be locked up irrespective of the verdict in the trial.

Defence barrister Stuart Littlemore, QC, asked Navin's treating psychiatrist what would happen to the accused if he were acquitted.

Dr Anthony Barker said Navin would not be released back into the community, but detained by court order in a locked mental health facility.

"It is difficult to say for how long it will be," Dr Barker said.

The psychiatrist has treated Navin since his arrest and incarceration at the Alexander Maconochie Centre in February last year.

Dr Barker has given evidence that he believes Navin suffers from a schizophrenia-type illness, which has been managed via medication.

He said Navin would likely suffer the illness for the rest of his life and his future depended on medication.

The psychiatrist told the court that Navin would likely suffer delusions again if treatment stopped, and a relapse could have serious consequences for the safety of himself and others.

Dr Barker on Tuesday told jurors that he did not believe Navin had faked his symptoms – which included hearing voices – in a bid to dodge a murder conviction.

He said Navin had been suspicious and guarded about his psychosis, whereas fakers usually exaggerated and fabricated, and would tell everybody about their symptoms.

The trial continues on Wednesday.