An argument over superfoods led accused killer Christopher David Navin to fatally target his former housemate, a court has heard.
Navin, 29, is on trial in the ACT Supreme Court for the murder of Nicholas Sofer-Schreiber.
Navin admits he fatally stabbed Mr Sofer-Schreiber on Boxing Day 2013, but has pleaded not guilty by way of mental impairment.
The court has heard Navin was diagnosed with psychotic depression in 2011, but said he had stopped taking his anti-psychotic medication two months before the alleged murder.
After his arrest, Navin claimed he believed he had been watched by "observers" who were part of a criminal gang, had heard voices, and thought his mind could be read, in the months before the attack.
Friends made the grisly discovery of his body and alerted police on the afternoon of December 28.
By then, Navin had driven to a family property in northern NSW where he burnt his bloodied clothes and two knives, the alleged murder weapons.
The Crown case alleges a soured friendship was Mr Navin's motive for the killing.
But Navin claims he had killed Mr Sofer-Schreiber because he received messages that the deceased had hired a hitman to kill his family.
The court on Friday heard from forensic psychiatrist Professor David Greenberg who examined Navin in May.
Professor Greenberg said Navin claimed he had linked an argument he once had with the deceased about superfoods to his grandfather's death in 2013.
He believed the death had been punishment for the squabble with Mr Sofer-Schreiber, who he believed worked with the observers.
But he told the psychiatrist that he did not intend to kill Mr Sofer-Schreiber until he received a number of "messages".
The messages included seeing an envelope stuffed with money with the words "Thanks Nicky" written on it, which he thought had been payment for the murder of his grandfather.
He told Professor Greenberg that on the night of the alleged murder, he had seen a book cover, which said "on the loose", which he interpreted to mean a hitman was at large.
Soon after, he became convinced Mr Sofer-Schreiber had been a danger to his family when he saw a piece of wire that looked like a noose and his mother had tightened jewellery around her neck as if she had been strangled.
The court heard that when Professor Greenberg had asked why Navin had killed Mr Sofer-Schreiber, rather than reporting his fears to police, the accused had responded: "It was the only realistic option."
Navin claimed, after the killing and as he drove north, he saw a car which read "9 out of ten", which he inferred as a score on the quality of the alleged murder.
He told Professor Greenberg that he had attended Mr Sofer-Schreiber's January funeral as it would have seemed unusual if he had not.
The trial continues on Monday.
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