The teenager made what a witness described as "prehistoric monster sounds" during a vicious attack, in which he killed Canberra watersports stalwart Richard Cater by stomping on the 82-year-old's head.

While "tripping" on LSD he had bought on the dark web, the killer also turned on two of the man's friends, leaving one with a fractured spine in the random March 2019 ambush outside Mr Cater's home in Palmerston.

It took five police officers to arrest the teenager, who cannot be named because he was 17 at the time in question, as he lashed out at them while in his drug-induced psychosis.

Despite having no memory of the events, he pleaded guilty in the ACT Supreme Court last year to charges of murder, intentionally inflicting grievous bodily harm and assault occasioning actual bodily harm.

Justice Michael Elkaim sentenced the offender to 15 years in jail, with an order that the balance of the term be suspended once the teenager had served eight-and-a-half years behind bars.

The judge accepted that the murderer had not meant to do what he did, but pointed out that the youngster had willingly taken drugs that had made him "descend into a raving and violent marauder".

The sentence upset the close-knit family of Mr Cater, whose son Mark told media it had brought them no closure and left them "most disappointed".

The killer, now aged 19, recently went to the ACT Court of Appeal to challenge the severity of the sentence.

He returned to that court via audio-visual link on Friday afternoon to be told by Chief Justice Helen Murrell his case had been successful.

Chief Justice Murrell, together with Justice David Mossop and Justice Tom Thawley, resentenced the teenager to 11 years and nine months in jail.

The trio of judges ordered that the new term be suspended after just four-and-a-half years.

Because the killer's sentence has been backdated to commence on the date of his arrest, he will now be released from prison in mid-September 2023.

The Court of Appeal's judgment reveals that it found Justice Elkaim made an error in the original sentence by setting the unsuspended portion of the total as if it were a non-parole period.

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The law in the ACT does not allow for the setting of a non-parole period for offenders who were under 18 at the time of their crimes.

This mistake meant the Court of Appeal was required to undertake the resentencing, which it did noting the particular importance of "individualised justice" for young offenders.

The three judges described the attack perpetrated by this offender as being "nightmarish" for the victims, but they noted he had not deliberately sought them out "as he roamed the streets in a psychotic state".

They added that his genuine remorse was apparent and that he was behaving well behind bars.

In one example, they said the teenager had shown "maturity and leadership" during the August 2019 Bimberi Youth Justice Centre riot.

"It must be deeply hurtful to the victims and their families that the horrific incident which caused their shocking losses has afforded the [offender] opportunities for educational and personal development that will enable him to lead a more productive adult life when released," the appeal judges said.

"However, the criminal justice system is not a vehicle for retribution.

"Especially in the case of young offenders, it should, where possible, be a vehicle for rehabilitation, an outcome that benefits both the individual offender and the broader community.

"The present case demonstrates the capacity of young offenders to rehabilitate."

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