The boy, then 15, broke into John Bourke's Maryborough home in July 2018, stomped on his head and bashed him to death.

The 45-year-old man, who had brittle bone disease, was completely innocent.

Crown prosecutor Grant Hayward on Friday said the murder was a "revenge attack" on someone the teenager mistakenly believed had committed a sexual assault.

"He was acting as a vigilante," Mr Hayward told the Victorian Supreme Court at a pre-sentence hearing.

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The 15-year-old, who cannot be named, is awaiting sentence after being found guilty of murder.

He and his friend, then 17, went to Mr Bourke's house from a nearby party.

The younger boy was intoxicated and had been involved in an argument relating to an alleged sexual assault.

He thought Mr Bourke was a paedophile but his accomplice later admitted "we got the wrong house".

Mr Hayward said the 15-year-old decided to attack the disabled man, while "simmering" at the party, but that the move wasn't spontaneous.

"This was a person who acted very violently to essentially an allegation he committed a sexual offence and an allegation someone else had committed a sexual offence," Mr Hayward said.

Mr Bourke was about 145cm tall and couldn't stand without crutches.

His brittle bone disease meant even a bump or knock could cause fractures, of which he suffered about 400 during his life.

Mr Bourke's heartbroken father, Peter, said police warned him against viewing his son's body at the crime scene, due to the extent of the injuries.

He described the murder as "sadistic", "brutal" and "unforgivable".

"I lay awake at night feeling guilty that I was not there to defend him," Mr Bourke told the court.

"I cannot comprehend how two young men could complete such a horrendous act against an innocent man.

"You only had to look at him to see he could not defend himself."

Defence lawyer Rishi Nathwani said the 15-year-old had been subjected to violence at home.

He added that no weapon or planning was involved in the murder but that the teenager recognised Mr Bourke died in "needless and tragic circumstances".

Justice Elizabeth Hollingworth said the boy had internalised violence as a means of resolving problems.

"The exposure to violence may trigger violence itself," Justice Hollingworth said.

The teenager will be sentenced at a later date.

- AAP

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