The jury has started to hear the closing addresses from the prosecution and defence teams in the trials of the two teenagers, who have pleaded not guilty to the murder of 45-year-old Mr Bourke.

The accused were aged 15 and 17 when Mr Bourke was killed in his Derby Road home in the early hours of July 15, 2018.

The prosecution argues the pair wanted to inflict really serious injury in the assault on Mr Bourke, but defence counsel for the teenagers deny this.

The jury will be asked to consider finding the teenagers guilty of manslaughter if they do not find them guilty of murder.

Crown prosecutor Grant Hayward said Mr Bourke - who had osteogenesis imperfecta, or 'brittle bone disease' - died because he suffered a severe beating that included being kicked and stomped on the head.

He said the jury could not be satisfied beyond reasonable doubt that the two accused intended to kill Mr Bourke, but a person was also guilty of murder if they intended to cause "really serious injury" to another and they died as a result.

"And that is what the prosecution is saying in this case, that there was intention to cause really serious injury," Mr Hayward said.

"And a person can be found guilty of murder, even if they didn't personally strike the fatal blow or blows."

Mr Hayward told the jury there was an understanding between the two accused that a person would be really seriously injured and they both participated in an assault that resulted in the death of a man.

This meant, he said, both teenagers were responsible for Mr Bourke's murder.

The trial so far:

Discussing the evidence against the teenagers, Mr Hayward referred to the evidence of witnesses who told the court they had had conversations with the accused or overheard them talking about the assault and potentially killing a man.

He also called into question the credibility of the accused and the accounts they gave police, saying their stories changed and sometimes contradicted the evidence of certain witnesses.

He told the jury they should reject any argument that Mr Bourke's bones, being brittle, could be broken easily without the intention of causing really serious injury because of medical evidence that "significant force" was required to cause some of the injuries Mr Bourke suffered.

Mr Hayward also said it did not follow that because Mr Bourke's bones could be broken more easily, his attackers held back.

"If you stomp on their face surely you are intending to cause really serious injury," he said.

The prosecution accepted that Mr Bourke was not the intended target of the assault, which was meant for another man, who was accused of sexual abuse.

Rishi Nathwani, defence counsel for the younger of the accused, told the jury the prosecution in this case had begun with the theory of murder and worked backwards to find evidence to fit their story.

Mr Rathwani said the prosecution case as to what their client was thinking at the time was circumstantial, and the only direct evidence of this was the accused's own police interview.

He referred to his client's plea at the beginning of the trial, "Not guilty of murder but guilty of manslaughter", and said this was what the teenager first told police.

Mr Nathwani said the version of events the teenager gave police in his interviews was consistent with later forensic and witness evidence, which showed he was telling the truth.

The teenager was also forthcoming with information about why the attack occurred, he said, which would not make sense if he was trying to downplay the situation.

Mr Nathwani addressed the jury about comments the boy made in his interviews, when he told police he did not want to kill the man and said: "That wasn't meant to happen".

"He's admitting manslaughter there and denying murder, and he does it from the beginning," Mr Nathwani said.

Mr Nathwani said the evidence of witnesses showed his client was in a good mood that night, and after an argument he was sad, not angry.

Witnesses' evidence also suggested the boy was upset after the assault, he said, not triumphant as one might expect of someone who achieved what they set out to do.

Mr Nathwani said Mr Bourke's health conditions meant he suffered injury more easily, and the forensic pathologist could not rule out that a healthy person would have survived.

The lawyer referred to the evidence of a witness that the teenager was a "caring kid", telling the jury he had made a "horrible, terrible, disgraceful error" but that "does not make him a murderer".

Defence counsel for the older teenager, Jarrod Williams, told the jury that not only did his client not form an understanding with his co-accused to really seriously injure Mr Bourke, but that the younger boy was solely responsible for killing Mr Bourke.

Mr Williams said his client quickly withdrew from the attack once he saw his co-accused stomping on Mr Bourke's head, and tried to stop the assault.

The teenager told police there was no agreement or understanding to cause really serious injury, Mr Williams said, adding that his client was also not helping his co-accused when the fatal injuries were inflicted.

He told the jury that the teenager cooperated with police and answered all their questions, which spoke to his honesty.

Mr Williams said the accused told police he had hit Mr Bourke, and stomped on his legs because he did not want to hurt his head.

There was a spontaneous understanding formed between the two accused when they went up the stairs to confront Mr Bourke, he said, but this was not to kill, really seriously injure or place Mr Bourke at risk of serious injury.

He said the older boy's attempt soon after to stop the assault was a clear indication that agreement had ceased, and this agreement ended before the fatal injuries occurred.

The teenager also told police that Mr Bourke was alive when he left to return to the party, he said.

Mr Williams drew attention to inconsistencies in the evidence of witnesses referred to by the prosecution, saying they were not reliable witnesses.

But the evidence of other witnesses supported his client's version of events, he said.

Mr Williams will continue his closing address on Tuesday, before Justice Elizabeth Hollingworth gives the jury instructions ahead of their deliberations.

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