Arthur Haines was asleep on the top floor of a friend's house ahead of a visit to Sydney's Royal Easter Show in April 1998.
But he soon became the victim of a bitter neighbourhood dispute in inner-city Waterloo when Gregory John Walker threw a Molotov cocktail into the home's kitchen.
"The results were catastrophic," Justice Hament Dhanji told the NSW Supreme Court on Tuesday.
Arthur became trapped on the third floor as the fire spread rapidly.
He was forced to jump from a window, landing smouldering and with severe burns on up to 65 per cent of his body.
The 13-year-old died in hospital 11 weeks later.
More than 27 years on, Walker kept his eyes downcast as he was sentenced to a maximum of 10 years and 9 months behind bars for Arthur's manslaughter.
The 58-year-old demonstrated a "callous disregard for the enormity of what occurred" in the aftermath of the fire, the court heard.
When a neighbour confronted him about the fire and said "I know you've done it", the then 30-year-old replied: "If you think that was a big fire, wait until you see my next one."
But Walker had since shown remorse, Justice Dhanji said.
In 2014, as police tried to mount a case against him, he told a witness he "wouldn't have gone through with it" if he knew children were inside the home.
Walker was arrested after NSW Police offered a $1 million reward for information in 2020.
He has an extensive criminal history with convictions dating back to the 1980s and was on parole at the time of the offending.
But evidence showed the former boxer had effectively turned his life around since his deadly actions in 1998, even establishing a not-for-profit for young people.
"There is an element of him attempting to make good on his past wrongs," Justice Dhanji said.
But the judge also noted Walker only admitted unlawfully killing Arthur in October, on the eve of a trial for the more serious charge of murder.
That was despite Arthur's family's obvious desire and lengthy wait for answers.
"While the ledger cannot be squared by the good done by the offender in recent times, that contribution must be taken into account and given weight," Justice Dhanji said.
With time served, Walker will be eligible for parole in February 2029.
Arthur's mother Julie Szabo, who ensured the public never forgot her son during repeated media appearances seeking answers, was surrounded by loved ones as the sentence was delivered.
She previously detailed the heavy guilt she has carried since allowing Arthur to have his first sleepover with friends.
"It was going to be the first night he had not slept under the same roof as me," Ms Szabo wrote in a statement earlier read out in court.
"I said 'yes' ... I think about that decision a lot.
"I gave him the biggest hug, we both said we loved each other, I didn't know at the time it would be one of our last hugs."
Justice Dhanji thanked Ms Szabo and acknowledged the pain felt by Arthur's family and the community.
"No sentence I impose can right the wrong that has been done," he said.
Justice Dhanji also took into account a charge of maliciously inflicting grievous bodily harm that Walker committed when he punched a neighbour and bit off part of his ear in April 1998.
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