The 20-year-old man, who cannot be named because he was underage at the time of his offending, wore a hooded jumper in the ACT Supreme Court dock on Thursday.

He pleaded guilty to a charge of importing a marketable quantity of MDMA between February 18, 2019, and March 1 that year.

The killer also admitted possessing a prohibited weapon, namely two knuckledusters, between two dates in March 2019.

It was one night that month that the offender, who had been buying LSD for just $4 per tab on the dark web, took some of that drug at a McDonald's and then at his home in Canberra's northern suburbs.

He told a friend he was "freaking out" and "tripping" before, as Justice Michael Elkaim put it, "he descended into a raving and violent marauder" as a result of his drug use.

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The youngster "fell into a psychedelic crisis" and went roaming in the night until he came across a carload of elderly people, who were pulling into the driveway of victim Richard Cater's home after a dinner outing.

He ripped open one of the car's doors and launched a frenzied attack, which included him biting the couple in the front seats and pulling the driver's head forward so hard he fractured her spine.

When "Dick" Cater, an 82-year-old back seat passenger, heroically came to the aid of his friends in the front, the drug-crazed teenager overpowered him and killed him by viciously stomping on his head.

One of the surviving victims later described noises the killer made during the horrific and random attack as "prehistoric monster sounds".

The offender, who was 17 at the time, fought with multiple police officers for nearly two hours at the scene before they managed to handcuff and control him enough that paramedics could sedate him.

He later claimed to have no memory of what had happened that night, but he nevertheless pleaded guilty in 2020 to charges of murder, intentionally inflicting grievous bodily harm and assault occasioning actual bodily harm. He also admitted possessing a prohibited substance and possessing cannabis.

Justice Elkaim sentenced him to 15 years in jail for those offences, with an order that the remaining time be suspended once he had served eight-and-a-half years.

The killer later successfully challenged the severity of that penalty, and was resentenced by the ACT Court of Appeal to 11 years and nine months in jail.

The time he would actually need to spend behind bars was almost halved to just four-and-a-half years as part of the resentencing.

Having been in custody since the night of the murder, he is currently set to be released from jail in September 2023.

This outcome left Mr Cater's family gutted, with his widow, Noelene, telling The Canberra Times she felt "very nervous" about the prospect of seeing the killer out and about in the near future.

Following his latest guilty pleas on Thursday, however, the young offender faces the prospect of having his time in custody extended beyond his scheduled release date.

He will be sentenced for the drug importation and weapon offences at a later date.

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