But Justice Hilary Penfold has again raised questions about flawed legislation requiring her to guess a sentence for murder on a series of hypotheticals.

Klobucar, 27, was found not guilty by way of mental impairment for the killing of Miodrag Gajic, 71, on New Year's Day in 2014.

Mr Gajic was beaten to death in his unit, suffering up to 10 blows to the head.

A jury found Klobucar to be the killer, but agreed with the evidence of three psychiatrists that he was mentally impaired at the time, and did not properly understand his actions were wrong.

That verdict means that Klobucar will be held in a secure facility until he is no longer deemed a threat to himself or the community.

But the verdict also forced the ACT Supreme Court into a peculiar position. It meant Justice Hilary Penfold was required to give an estimate of how she would have sentenced Klobucar, had he been found guilty of the murder.

That meant the court was forced to pretend that Klobucar's mental illness - which would play a major role in sentencing him - wasn't serious enough to give him a mental impairment defence.

As Justice Penfold put it, the court was being asked to determine a "hypothetical on a hypothetical".

The decision on how long Klobucar can be held is up to the ACT Civil and Administrative Tribunal, which will assess whether he is a risk of harm to himself or others in the community.

Klobucar's case will be reviewed monthly until it is determined he no longer poses a risk.

Justice Penfold gave an estimate of a 20-year sentence, reduced to 17 years and nine months when Klobucar's time already served was taken into account.

That estimate will be used as the "limiting period", or maximum time that the ACAT can detain Klobucar for, although it could conceivably hold him for longer if it wished.

Justice Penfold described the killing as a "brutal and pitiless attack on a frail old man".

Klobucar's mental illness had deteriorated significantly in the lead-up to New Year's Day 2014.

He became obsessed with paedophiles, believing himself to be in a secret police unit targeting underground child abuse networks.

Those older than him quickly came under suspicion, and he often became violent and aggressive toward such individuals.

It was in that state that he met Mr Gajic, two days before he beat him to death.

Klobucar had gone with his uncle to buy cannabis from Mr Gajic at his Phillip unit.

Later, he noticed Mr Gajic's car, a Nissan Micra, remarking that it was a "gay car" and that the older man was probably a paedophile who had DVDs of him from his childhood.

He pestered his uncle for the address a day later, before returning to the Mansfield Place complex between 10.40am and 11.10am on New Year's Day.

Klobucar was arrested for an unrelated disturbance at the Lighthouse pub in Belconnen later that night.

An intelligence officer who was looking into Mr Gajic's killing happened to see a photo of a shoe found in Klobucar's ute after the arrest at Lighthouse.

He connected that shoe to bloody shoeprints found at the scene.

During Klobucar's month-long trial, the Crown relied on what it said was an overwhelming circumstantial case proving he was the killer.

The Crown's evidence included the bloody shoeprints, a small amount of Mr Gajic's blood found on Klobucar's shoe, a timeline placing Klobucar in the unit at the time, and phone calls the killer made from the victim's phone immediately afterwards.

Klobucar denied he was responsible for Mr Gajic's death.

He argued the older man was already dead when he got there, and that he'd panicked, walked through the home, leaving shoeprints, and grabbed Mr Gajic's phone with the intention of calling police.

The jury, through their verdict, rejected Klobucar's account.

The ACT Civil and Administrative Tribunal began considering Klobucar's case at a hearing on Wednesday.