A Sydney office furniture salesman who allegedly shot dead a Victorian man in a campervan was mentally ill at the time and believed he was a secret agent for the Queen, a court has heard.
A forensic psychiatrist told Justice Robertson Wright that Simon Slavko Stojic was experiencing paranoid and grandiose delusions when Brett Jardine was gunned down in the middle of the night at Kingsgrove in July 2017.
Professor David Greenberg said Stojic had some knowledge that murder was "legally wrong" but because of his severe mental illness he didn't know it was "morally wrong".
According to crown documents, Stojic made numerous paranoid and bizarre statements concerning conspiracy theories to friends and colleagues in the months leading up to Mr Jardine's killing.
"My diagnosis was that Mr Stojic suffers from a severe mental illness, namely a chronic delusional disorder," Prof Greenberg told the judge-alone trial in the NSW Supreme Court on Wednesday.
The psychiatrist said at the time, Stojic believed he was a secret operative for ASIO, the CIA and the royal family.
"He had a grandiose belief system about the royal family," Prof Greenberg said.
"He was a special agent for the Queen, he was undertaking missions for her (and) she had an aeroplane ready at Bankstown Airport for him if the need arose to fly him on this mission."
The court heard the accused had told colleagues he was Princess Diana and Prince Harry's bodyguard.
Following his arrest, he told police that former prime minister Julia Gillard had watched on while a family member was tied to a horse.
He'd previously told a colleague "the Queen and the Vatican are at war" and both the CIA and the Rothschild family wanted to kill him.
Prof Greenberg concluded that due to his psychotic delusions, Stojic believed his life was being threatened by Mr Jardine, and he was eligible for the defence of mental illness.
"One hundred per cent, no doubt he suffers from a severe mental illness."
Mr Jardine travelled to Sydney in June 2017 to work on the WestConnex road project.
He was staying in a campervan at a Kingsgrove car park which was on the same street as the offices of Statewide Office Furniture. Stojic was the director of that company.
Prosecutors say CCTV shows Stojic was inside the business in the early hours of July 26 and exited at 2.25am.
Seven minutes later, Mr Jardine called triple-zero to report a man hanging around his campervan and kicking the door.
Stojic is accused of firing into the side of the campervan several times before shooting Mr Jardine in the left shoulder and abdomen.
According to court documents, Stojic later told a sales and accounts manager at Statewide Office Furniture he'd killed Mr Jardine but claimed it was a "sanctioned operation".
"You know what's going to happen when ex-military and a federal agent stand-off against each other - one of them is going to end up dead," he allegedly told his colleague.
"So I said to him 'either f*** off or I'll shoot you'. He said 'No'. So I shot him."
The trial before Justice Wright is expected to conclude with closing statements on Thursday.
Australian Associated Press