He then walked into his parents' bedroom and did the same to his sleeping father.

But why the now-48-year-old "passive" and "non-violent" programmer turned homicidal in the Sutherland home is still yet to be fully explained, the NSW Supreme Court was told on Friday.

"The act was entirely unpredictable, unanticipated, unforeseeable ... and utterly out of character," defence barrister James Glissan QC said.

Reid is being sentenced for manslaughter after prosecutors accepted he had a partial defence to murder due to substantial impairment caused by an abnormality of the mind.

Medical experts agree he has depression but can't agree on what other psychiatric conditions he has, and therefore what treatments he will need.

Mr Glissan, who acknowledged the crimes could only be viewed as very serious and terrible, said Reid's former colleagues had effectively described the software developer as "passive, non-violent and not ever, in any way, aggressive to anyone".

"That is entirely supported by every one of the statements tendered by his family," the barrister said.

"He'd never exhibited any tendency to violence."

Noting Reid had appeared truthful in interviews and shown a desire to understand the events, forensic psychiatrist Stephen Allnutt said the motive currently appeared "inexplicable".

"While he gives the motive, the motive seems to be not being happy with his parents and that being the cause of these problems," he said.

"It's not the way most people would deal with that. They would just move out of home."

Dr Allnutt said a constellation of symptoms pointed to underlying cognitive issues but the fundamental driving factor was unknown.

Depression had caused the "very intelligent man" to have a nihilistic view of his circumstances and his parents, who he remained dependent upon, the expert said.

"I get the feeling there was a strain in that relationship," Dr Allnutt said.

"His response to that seems to be distorted and that's a combination of depression and a neurological cognitive problem."

In the eight years to the 2019 attack, Reid went from a highly regarded employee to someone sleeping at work, Mr Glissan said.

He'd also posted half a dozen times online about suicide and attempted to take his own life after killing his parents.

Shortly after killing his parents, he told police: "I thought I had a good relationship with my parents, obviously I don't."

Reid later nominated one of his parents as his next of kin - a clear indicator of his mental issues, Mr Glissan said.

The Crown accepted there was no element of subterfuge in any of Reid's discussions with triple-zero operators, police or medical experts.

But Justice Peter Johnson was urged to ensure any sentence accounts for Reid's ongoing risk.

"If he has a partner or gets involved in a close way with other people, who's to say this couldn't happen again," deputy senior crown prosecutor Mark Hobart SC said.

"There was no planning whatsoever.

"He just got up in the morning and killed his parents. It's quite frightening."

Justice Johnson said there was no dispute about the magnitude of the mental health condition affecting Reid in August 2019.

Reid will be sentenced on May 7.

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Australian Associated Press