Paul Gary Turner, 43, stabbed 33-year-old Sarah Marie Thomas six times, including three times in her neck, at Joondalup Justice Complex on December 20, 2016.

One of the stabs cut the mother-of-two's carotid artery, which killed her within seconds.

She also suffered a slash across her left index finger, which a forensic pathologist described as a typical defensive wound.

Administration worker Crystal Sudholz told Turner's Supreme Court trial she heard muffled yelling then a "horror movie scream" before the registrar ran out of the conference room, shouting for security and an ambulance.

Turner then walked out into the waiting area, slid against a wall to the floor, put his hands in his lap and "stared into thin air", Ms Sudholz testified.

The pair had been embroiled in an increasingly tense dispute over custody of their children and the mediation session was about his claim she owed him money.

Moments before the attack, Ms Thomas declined to make an offer, but the meeting was cordial.

Turner claimed that after he had an accident while working as a truck driver in 2015, he suffered symptoms including intense headaches and seizures that caused blackouts.

He claimed he had no memory of what occurred when he blacked out and the next thing he recalled that day was being at a police station.

"I remember looking at my hands and they're covered in blood," Turner testified, crying.

On Tuesday, Supreme Court Justice Joseph McGrath labelled Turner's claims he was in a dissociative state at the time of the murder "a concoction" but accepted he had in the past suffered from "some form of dissociative seizures".

Turner's defence counsel Lisa Boston conceded there was an intention to kill, but said it had only been planned that morning following mounting stress and long-term depression.

Justice McGrath said he was unable to find there was any planning before that.

"The evidence is compelling that you went to the courthouse with the intention of killing the deceased," he said.

Having taken a knife from home and hidden it either in a folder or in his shorts, he then calmly inflicted the lethal wound, targeting the carotid artery as he has been extensively trained to do, knowing it would lead to certain death.

"When the deceased was vulnerable, you murdered her," Justice McGrath said.

He said Turner appeared to have shown limited remorse.

Prosecutor James Mactaggart said Turner had a smug look on his face in the moments after the attack, having taken the opportunity to "make a very public statement".

"This was a crime of quite outrageous violence," Mr Mactaggart said.

Ms Thomas' family wept and said "yes" after hearing Turner's term, while he displayed no emotion.

Australian Associated Press