Supreme Court justice Betty King said the murders committed by Thomas Hemming had been bizarre, inexplicable and incomprehensible and had sent "a shudder of fear through our community".

"Your crimes were horrific, they were motiveless, a desire to kill  just to see what it was like," Justice King told Hemming, 21, on Friday after he had pleaded guilty to murdering Robert Adamson, 64, and his wife, Cheryl, 60, on February 19.

"People rightly believe they are safe in their own homes, that their home is their castle and your actions have all but shattered that belief for a number of people."

Hemming did not know the Adamsons, who had two children, Michael, 28, and Katie, 30, but had seen them in the local area walking their dogs when he decided to act out his fantasy to find out what it was like to kill someone.

He walked about 160 metres from his home to the Adamsons' house in Omama Road at about 6am, knocked on the couple's front door and asked to use their phone.

The Adamsons, who were both wearing pyjamas, invited Hemming inside in the spirit of being good neighbours.

Hemming, armed with a Cold Steel Marauder knife he had ordered online, followed the couple down the hallway before attacking Mr Adamson, an accountant, in the living room area, stabbing him repeatedly. Mrs Adamson, a librarian at Melbourne Grammar School, hit Hemming over the head with a broom trying to save her husband of 33 years.

Hemming then stabbed Mrs Adamson in the neck, chest and back before walking home and hiding his bloodied clothes under his bed. The knife was found embedded in Mrs Adamson's upper right shoulder and neck area.

After his arrest, Hemming told police he felt that morally it was better to kill an old couple than someone young, that he had found himself thinking of killing people over the past few months, and that  on the night of the murders he was drunk and impulsive and didn't have the common sense not to go through with his fantasy.

"I just kind of find myself thinking about killing people," Hemming told police.

"Just don't even realise ... in no specific ways."

Justice King described the autopsy reports on the Adamsons as "chilling" as they showed "a degree of ferocity in the blows inflicted, in that they were deep and penetrating, as well as numerous".

The judge said Hemming, a former De La Salle college student who has the autism spectrum disorder Asperger Syndrome, had exceedingly poor prospects of rehabilitation.

"You know what you did was wrong but there is nothing to indicate that you would not do it again as you lack an emotional connectedness with people in general," Justice King said when jailing Hemming for 32 years with a non-parole period of 27 years.

Psychiatrist Dr Daniel Sullivan did not believe Hemming was a psychopath or sociopath but that he had a lack of empathy and the risk of him re-offending was "unknowable".

The judge said Hemming's Asperger's, which was not normally a syndrome that caused people to be violent, did not reduce his moral culpability.

Hemming remained incapable of feeling genuine empathy, sorrow or regret for his fellow human being.

"The result (of Hemming's actions) is that two decent, caring, helpful people who were asleep in their own home have been woken, come to the aid of a neighbour and been brutally murdered.

"It is to be hoped that with time, medical science will find some method of assisting you to find that empathy and humanity that you currently lack, be it with medication or some form of psychotherapy, but at the moment it does not exist."

Asked by police if his fantasy had lived up to what he thought it would be after he had killed the Adamsons, Hemming replied: "Not really, no.

"Just cos I, it's strange because I don't really remember it too well or anything and you know, in a way it kinda feels like I didn't do it ... if that makes sense.

"I remember the kind of vision of it, like kinda seeing it a bit but I don't really remember how I was feeling or what I was thinking of or anything at the time."