A Supreme Court judge must decide whether an axe murderer's brain injury lessens his moral culpability below that of his co-killer over a horrific attack on a 14-year-old boy with autism.

In separate trials this year Darren Wilson and Joel Henderson were found guilty of murdering Timothy "Timmy" O'Brien in Scarsdale, near Ballarat, on January 5, 2013.

Wilson was in July sentenced to spend 30 years and six months in jail. Henderson appeared before the Supreme Court on Friday for a pre-sentence hearing.

Previous court hearings were told Henderson hit Timmy with the blunt edge of the axe and told him "Die, c---, die", before Wilson struck the teenager with the sharpened edge of the axe.

Timmy initially hit Henderson on the head with a baseball bat after going to help stepfather Peter Williams, who had been lured to a farmhouse by Wilson and Henderson, who planned to bash him. Wilson and Mr Williams are cousins who had a falling out.

Defence counsel George Georgiou, SC, told Justice Bernard Bongiorno on Friday that Henderson's moral culpability was lesser than Wilson's because he was unable to control his temper after being hit with the bat.

Mr Georgiou said his client was impaired by an acquired brain injury he suffered in a suicide attempt more than a decade ago when his partner left him.

"Mr Henderson, by reason of the acquired brain injury, is a person who's not able, according to [psychologist Jeffrey] Cummins, to think clearly and not able to exercise appropriate judgment," he said.

"Had he had those capacities his actions may have been different on this night."

Prosecutor Christopher Dane, QC, described Henderson as a "simpleton" who lost his temper, but said trying to distinguish between the killers' culpability was near impossible.

"To divide up the moral culpability of these two men who have taken an axe to a boy is an attempt to divide angels on the head of a pin," Mr Dane said.

Henderson has spent more than 1000 days in custody awaiting his fate and, the court heard, had found custody difficult because of his brain injury and mental-health problems, and a previous assault, which meant he was now in protective custody.

The court heard Wilson and Henderson took the axe to and from the property.

Lisa Trezise, who drove the pair to and from the scene, was last year sentenced to spend 21 months in jail, but that term was wholly suspended.

Henderson, a father of four, was supported in court by his mother.

Justice Bongiorno is expected to sentence Henderson next week.