Murder accused Joshua Higgins' anxiety went "through the roof" when he saw his alleged victim holding a knife and giving a "death stare", a court has heard.
Mr Higgins took the stand in his defence on Wednesday, telling the jury he had woken up on the morning of March 11, 2019, to find his shorts and underwear pulled down and his penis exposed with Jae-Ho Oh on top of him.
He said he confronted the 56-year-old and then grabbed his phone and started trying to record a confession that the older man had sexually assaulted him.
The accused murderer said Mr Oh then ran to the kitchen and brought back a knife. "I remember having a massive panic attack," Mr Higgins said.
He said they struggled and he bit the man's arm to try and disarm him. They rolled off the bed and Mr Higgins said he managed to stab him in the back.
He said they struggled again in the kitchen and he stabbed Mr Oh in the neck.
He said he did not intend to kill Mr Oh. "It's absolutely shattered me," he said.
Mr Higgins, 32, was coming down after injecting ice, had not slept for 60 hours and had drunk 30 standard drinks when he stabbed Mr Oh, 56, multiple times in his Gungahlin townhouse.
He has pleaded not guilty to murder.
The ACT Supreme Court jury also heard from a forensic psychiatrist who examined Mr Higgins.
Dr Stephen Allnutt told the court after hearing a police audio recording of Mr Higgins shortly after the stabbing that he did seem psychotic.
But the forensic psychiatrist said it was difficult to disentangle the cause of Mr Higgins psychosis.
A key question for the jury will be whether Mr Higgins' actions were caused by the alcohol and drugs he had consumed, or his underlying mental health.
Mr Higgins has a history of mental illness including PTSD.
Dr Allnutt agreed with defence barrister Kieran Ginges that a person with PTSD could have a heightened response to a perceived sexual assault.
The trial continues before Justice John Burns.
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