Now 17, he faced the NSW Supreme Court on Monday for the first day of his trial after pleading not guilty to the murder of Chi Liao, 30.
The teenager - who cannot be identified for legal reasons - is accused of stabbing the Taiwanese national through the sternum and into his heart during a botched drug deal in Sydney's west around midnight on July 19, 2024.
He had contacted Mr Liao on an encrypted platform to purchase drugs but did not have enough to cover the $430 payment when the man arrived, crown prosecutor Craig Evans said in his opening statement.
The 15-year-old allegedly planned to seriously hurt Mr Liao when he pulled out a 69cm multi-edge knife - known as a zombie blade - and stabbed him.
The teen brandished the weapon to obtain the drugs he could not afford, the prosecutor told jurors.
The court was told the teen rushed home and told his mother he had used the machete to scare off the other man, who put himself in the way of the weapon.
The 15-year-old told police he "f***ed up" when he told them where to find the weapon, later adding "this was all just over a bit of weed".
His barrister Sarah Talbert told the jury that the main issue they needed to determine was what was going on in her client's mind at the time of the altercation.
There is no dispute that the teenager lived with multiple serious mental health impairments in July 2024, including schizophrenia, post-traumatic stress disorder and depression.
Ms Talbert said the 15-year-old had at one point been bullied so badly that he wound up in hospital, leaving him mistrustful.
"He was a boy who, although he tried to act tough, was in fact afraid of the world," she told the jury.
"He had a terribly large knife ... and he kept it with him in the form of some kind of security blanket."
Ms Talbert told the jurors they would need to consider whether his mental impairment was so substantial that it would reduce his responsibility for Mr Liao's death from murder to manslaughter.
While CCTV footage from nearby homes will show the teenager holding the knife that delivered the fatal blow, Ms Talbert alleges he did not intend to cause serious harm.
It will fall to the prosecution to rule out the possibility that the 15-year-old acted in self-defence, his barrister argued.
The prosecution plans to do so, with Mr Evans using his opening statement to paint the teenager as the aggressor who produced a weapon in order to obtain drugs.
The trial continues on Tuesday.
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