Emotions ran high in the ACT Supreme Court on Monday as 16 family members and friends of the murder victim, Richard Cater, faced the 82-year-old's killer and read powerful victim impact statements.

The killer, who cannot be identified because of his age, has admitted to murdering "Dick" Cater and seriously assaulting two of the man's elderly friends during a random, drug-fuelled attack outside Mr Cater's Palmerston home in March 2019.

He was "tripping" on LSD and making "prehistoric monster sounds" during the attack, which has been described as the product of "drug experimentation gone wrong".

When the now 18-year-old offender's sentencing proceedings began on Monday, Mr Cater's widow called for the killer to be "punished to the full extent of the law".

Tragically, she and Mr Cater had been just a few weeks shy of their 60th wedding anniversary when the attack took place.

"The ferocious, animal-like actions of a complete stranger ... have caused me to be without my life partner," she said.

"The loneliness and the fear just never goes away."

Mr Cater's widow told the killer she hoped he would go through hell "because you certainly put me and my family through hell".

Statements were also delivered by the living victims of the attack, a couple whose names are suppressed.

One of them, whose spine was fractured by the teenage assailant, described Mr Cater as "a gregarious and decent man" who had been a great friend.

She also recalled the horror of the incident, saying: "The sense of helplessness ... will stay with me forever."

Her husband, who was ferociously bitten during the incident, praised Mr Cater as a selfless person.

The man said it was not lost on him that Mr Cater had been killed trying to defend his friends against the rampaging teenager.

Mr Cater's children and grandchildren heaped praise on the devoted family man in their statements, with descriptions including "true hero" and "bloody legend of a man".

They also gave harrowing accounts of the terrible impact his death has had on them.

One son, Michael Cater, described being haunted by a "constant vision" of his father lying "bloodied and barely recognisable" in a gutter after the incident.

"It eats away at you like a cancer," he said.

Several members of Mr Cater's family expressed particular anger that their loved one had died because of one person's "selfish" decision to take illicit drugs.

While the offender was only 17 at the time of the attack, they believed he had committed "very adult crimes" and should be punished as such.

When the killer was called to give evidence, he apologised to the family and said he understood why they must hate him.

He described his drug experimentation and its consequences as "a mistake I will regret for the rest of my life".

"I hope that one day you can forgive me and know I feel truly sorry," he said.

The murderer's barrister, Ken Archer, said the teenager felt "a deep sense of sorrow".

He also stressed that the teenager's "terrible crimes" were committed during a psychotic episode caused by drug use, and that the killer had "acted in a way that bears no relationship to the person he actually is".

The teenager, he argued, "could not have reasonably foreseen" the tragic consequences of the decision to take illicit drugs.

Mr Archer said his client had been "a model detainee" during his time in custody so far, and wanted to "achieve some form of limited redemption" upon his release.

Director of Public Prosecutions Shane Drumgold SC told the court the offender may well have good prospects for rehabilitation, but he needed to be punished for "grotesque" crimes that had caused "unbelievable agony".

"Whatever the circumstances, [the teenager's drug use] resulted in the most horrific violence and murder," Mr Drumgold said.

He said it was particularly egregious that the victims were elderly and Mr Cater was outside his own home when the group was set upon.

Justice Michael Elkaim will sentence the killer on Wednesday.