Cassius Turvey, a 15-year-old Noongar Yamatji boy, died in hospital 10 days after prosecutors say he was chased, knocked to the ground and "deliberately struck to the head" in Perth's eastern suburbs on October 13, 2022.
Aleesha Louise Gilmore, 23, her boyfriend Jack Steven James Brearley, 23, and his mates Brodie Lee Palmer, 29, and Mitchell Colin Forth, 26, are on trial, charged with murdering Cassius.
In police video of his arrest shown to the WA Supreme Court on Wednesday, Brearley claimed a group of young people with knives got off a bus and were headed to the home he shared with Gilmore and her family.
Arrested on suspicion of assault causing bodily harm and unlawfully wounding Cassius, he told officers: "I was attacked first".
"He stabbed me first three times across my leg," he can be heard saying in the police body-worn camera footage played for the jury.
"I stepped on his head and no pole involved or nothing."
Brearley told the officers the teen "was on the ground and he was just slashing my legs".
"Yeah I lost the plot but he had a knife and was stabbing me."
In a later police interview, Brearley told detectives there were 20 armed "kids" mouthing off at him and he ran at them.
"One tripped over ... and his mate threw him the knife and he got the knife and my foot was like this on his head ... and I just punched like three times," he said.
"Another c*** was ... running towards me so I ran back to the car."
Brearley also said a group of people had threatened to "run through" his home, break his bones and "bash my missus".
"What am I supposed to do? ... He pulled a knife on me ... He stabbed me and that's his fault," he said as he repeated his version of events, which changed as the interview wore on.
"I will admit I did punch him, I punched him max twice while my foot was on him."
Brearley said he, Forth and another mate had followed the bus in his friend's "monster" Ford ute
Asked who the other friend was he said: "I'd rather not say. He's a scary person".
Asked repeatedly about the group of young people, Brearley said: "They were just waving bats and poles saying 'come on, we're ready, we're ready and I was like Mitchell, you better help me if they come to my house".
"All these kids start pulling out weapons and s*** and start running in the bush towards my house.
"Then I got out of the car and every single kid ran."
Brearley said he chased "pretty much the fat one and he just stopped ... I pushed him".
Brearley described landmarks in the area where the incident happened, saying: "On one side it's Aboriginal house, Aboriginal house, Aboriginal house".
He then returned to the incident.
"His mate yells 'chuck me the knife' and his mate chucks him the knife and I said 'are you going to use it'?" he said.
"If someone is gonna mob my mates I'm gonna stand by my mates."
Prosecutors allege Brearley struck Cassius in the head twice with a metal shopping trolley pole while "hunting for kids" because somebody had smashed his car windows.
They allege Forth, Palmer and Gilmore helped him and knew his intent before the incident.
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