Cassius Turvey, a 15-year-old Noongar Yamatji boy, died in hospital 10 days after he was attacked in Perth's eastern suburbs on October 13, 2022 while returning home from school.
Brodie Lee Palmer, 29, Mitchell Colin Forth, 26, Jack Steven James Brearley, 23, and Aleesha Louise Gilmore, 23, have denied murdering him.
On the first day of an estimated 11-week trial attended by Cassius's mother Mechelle Turvey, prosecutor Ben Stanwix told the jury the group chased Cassius down.
"Caught, knocked to the ground and deliberately struck to the head with a metal pole," he told the West Australian Supreme Court on Monday.
"Set upon by people that he didn't know."
Mr Stanwix said Cassius suffered severe head injuries, including brain damage, which caused severe bleeding and ultimately his death on October 23.
He told the jury his death was "the end point of a complex series of events" that had absolutely nothing to do with Cassius".
They started on October 9 when Forth, Brearley, Gilmore and another man Robert MacKenzie, 20, allegedly unlawfully detained two teens, punching, kicking and stabbing one of them.
The incident happened as the group drove around in Brearley's Kia searching for a group of boys they believed had threatened to "bash" Gilmore's younger brother amid tensions over a girl and social media exchanges about the pair fighting.
While searching for one of them, the group allegedly came across a 14-year-old boy who they believed could help them find the boys who threatened Gilmore's brother.
MacKenzie allegedly pulled out a 35cm knife and forced him into the car before driving off.
The group then came across some girls at a bus stop, including Gilmore's younger brother's girlfriend.
One of them "yelled something like if they lied would get their heads punched in," Mr Stanwix said during his opening submissions.
Cassius was playing on a scooter when the light blue Kia hatchback pulled up beside him.
Brearley, MacKenzie and Forth jumped out and chased him down an alleyway as he attempted to flee, Mr Stanwix said.
They allegedly caught the teen outside a home and attacked him.Â
MacKenzie stabbed the boy in the chest, causing a shallow wound because it was still in its sheath with just the tip sticking out.
The home owner attempted to intervene but Forth told him to "get out of it dog, walk away".
CCTV footage of the incident shown to the jury showed a wide-angle view of a modest suburban street, with the group in the top left corner.
Mr Stanwix said Brearley could be heard demanding to know "where the f*** are they" in reference to the boys he believed had threatened his girlfriend Gilmore's brother.
"Then Brearley adds, seemingly without a hint of irony: 'You want to bully little kids'," he said.
The teen was dragged down a street before also being forced into the car, where Gilmore's brother told the men their alleged victim wasn't involved.
Two days later, police searched MacKenzie's home where they found the knife on a bedside table and him hiding under the bed.
Brearley, Forth, Gilmore and MacKenzie, who is also on trial but not for the murder charge, have previously pleaded not guilty to charges stemming from the alleged incidents of October 9.
These include unlawfully detaining two teens and assaulting one of them.
Brearley and Palmer have also denied stealing a hat and crutches on October 13. Brearley, Palmer, Forth and Gilmore also denied assaulting another person on that day.
The trial, which is expected to hear from 89 witnesses, continues.
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