Declan Cutler was alone walking down a dark street when he was kicked, stomped on and stabbed by a group of eight boys in March 2022.
He had just left a party in Reservoir, Melbourne's north, and had become separated from his friends.
In the early hours of March 13, a stolen Mazda pulled up and the group of boys got out and pounced on Declan.
In a two-minute frenzied attack, the group stabbed and kicked him as he lay on the nature strip.
The group got back in the car and drove off, but returned briefly to stomp on him again and remove his shoes, before leaving.
Local residents were awoken by yelling and shouting outside their homes and called triple zero.
Declan bled to death on the ground after suffering 152 injuries, including 56 stab wounds and 66 blunt force injuries.
The youngest of the group, who was aged 13 at the time, admitted stomping and kicking Declan while he was being stabbed, but pleaded not guilty to murder or manslaughter.
The boy, who is now 14, faced a judge-alone criminal trial in July and August of this year.
Prosecutors had to rebut the Doli Incapax legal principle, which presumes children under the age of 14 lack capacity to be held criminally responsible.
They argued the boy should be held responsible because he knew his actions were seriously or morally wrong.
But Supreme Court Justice Rita Incerti on Wednesday rejected this and found there was a "reasonable possibility" the boy did not know his offending was "seriously wrong in a moral sense".
"As such, this leads to the conclusion that (the boy) cannot be found guilty of murder and the alternative charge of manslaughter," she told the court.
She said the boy's cognitive and moral development suggested he "lacked agency", citing evidence from a child forensic psychiatrist who found he had "a limited capacity for autonomy" and was vulnerable to influence by anti-social peers.
Declan's supporters, wearing "Justice for Declan" shirts, cried from the court's public gallery as she handed down her verdict.
"It's disgusting," one person yelled.
Outside court, his father Bryan Beattie hugged Declan's best friend and said the judge made a "really bad" decision.
"It's not enough," he said.
"She could have just made a bad decision that judge and I hope she's happy with her decision."
The boy will walk free from court later on Wednesday.
Three members of the group were jailed on September 12 for their roles in the killing.
A 16-year-old boy, who was the second youngest, was jailed for up to 15 years after pleading guilty to murder.
Two 18-year-olds were handed youth detention sentences of three-and-a-half and four years.
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