Cassius Turvey, a 15-year-old Noongar Yamatji boy, died in hospital 10 days after he was allegedly struck in the head with a metal pole in Perth's eastern suburbs on October 13, 2022.
Jack Steven James Brearley, 23, Brodie Lee Palmer, 29, Mitchell Colin Forth, 26, and Aleesha Louise Gilmore, 23, have denied murdering him.
On the second day of an estimated 40-day trial, prosecutor Ben Stanwix told the jury they should only assess the evidence heard in the court when coming to a verdict.
He said many people had previously said the alleged murder was racially motivated but they had not "heard all the things you will see and hear".
"What a luxury for them ... but that cannot happen here in this courtroom," he told the West Australian Supreme Court on Tuesday as he finished his opening statement.
"Let me make this clear it is not the state's case it was racially motivated."
He conceded the jury would hear racially motivated evidence, such as people saying: "Black this".
Brearley's lawyer, Simon Watters, outlined his defence case, alleging that Cassius had "slashed" his client's leg after he chased the boy into bushes.
He told the jury that Cassius was with a "large mob" of about 20 teenagers, some as old as 18, and the pair had struggled on the ground, describing the boy as six feet tall and 110 kilograms.
"He noticed a little kitchen steak knife in his hands," he said.
Mr Watters alleged that Brearley, who described Cassius as the "fat one" to police, tried to run but the teen had a hold of his clothes and slashed his leg three times with the knife.
"Mr Brearley called out to Brodie Palmer: 'Help I've been stabbed'," he said.
Brearley punched Cassius in the face and put his foot on his head so he couldn't get up, as he previewed the evidence the jury would likely hear during the trial.
"He turned and saw Mr Palmer standing there. He was older, bigger and meaner," he said.
Mr Watters said that as Brearley "limped" away he saw Palmer allegedly strike Cassius in the head area three times with a metal pole taken from a shopping trolley.
"He heard Cassius scream and yelled for Mr Palmer to stop," he said.
"If it was Cassius who had the knife it takes the air out of the prosecution's tyres."
Mr Watters said Brearley, who he described as a "spindly and skinny kid", was at the scene because he wanted to "confront an angry mob" to protect his home and his girlfriend's younger brother.
He also said the jury would hear Palmer later admitted to Brearley that he had struck Cassius and allegedly threatened his client that if they told anyone "he and Gilmore would leave in body bags".
"There are alternative explanations consistent with the evidence," he said.
The prosecution alleged that Brearley stuck Cassius in the head with a metal shopping trolley because somebody had smashed his car windows a day earlier.
It said Forth, Palmer and Gilmore helped him and knew what his intent was.
The trial continues.
