Mathew Flame then woke up "under the control" of something and became convinced his mate, Liam Anderson, was "evil".

"It was so real that he was evil and he was there to get me," the then-20-year-old said in a recording played to the NSW Supreme Court on Monday

The man from Narraweena, on Sydney's northern beaches, has pleaded not guilty by reason of mental impairment to the bashing murder of Mr Anderson, the son of Angry Anderson, the lead singer of hard rock band Rose Tattoo.

The Crown contends Flame, now 22, was psychotic at the time due to his voluntary use of drugs, while the defence says he was labouring under the severe mental illness of schizophrenia.

The jury on Monday watched the accused man's taped police interview, given the day after Mr Anderson's death in a park near Freshwater beach on November 4, 2018.

Flame had walked to the park, accompanied by his mate, from a Queenscliff home after having heart palpitations, losing consciousness and believing he would die, he told police.

"We were just sitting there and that's when everything just ... what happened, happened," he said.

"In my head, there was a demon and I had to f***ing kill it."

Flame said he couldn't control himself and "could see it in his eyes that something was f***ing there".

"When I started attacking him, it was like he was a demon and I was like an angel," he told police.

A tourist has testified to seeing an emotionless assailant repeatedly stomping on the bloodied head of his victim.

Flame shed tears several times during the interview, aggravating remnants of capsicum spray used to subdue him the day before.

"Why would Liam be evil? He was my best f***ing friend," he said.

"It was like I was in, you could call it Hell."

Flame said he believed police arriving on scene were also demons out to get him.

He said the delusion was his first ever and had begun after taking the last of five caps of MDMA or ecstasy for the night.

He'd consumed cannabis, alcohol and about one gram of MDMA over the night, he said.

"It's all the chemicals, just a weird reaction," Flame said.

Mr Anderson had mentioned "I'm never going to leave you, man" as they walked to the park, but Flame couldn't recall any words during the attack.

"We were best mates five minutes before I overdosed and then I woke up in a completely different head space, and that's the truth," Flame said.

The jury has heard the accused began showing symptoms of a schizophrenic disorder seven weeks after his arrest.

The trial continues.

Australian Associated Press