The trial of Danny Klobucar, 27, for the alleged murder of Miodrag Gajic, 71, began in the ACT Supreme Court on Wednesday.

Mr Gajic was bashed to death at the front door of his Mansfield Place unit in Phillip on New Year's Day in 2014.

Klobucar has denied killing Mr Gajic, but is also relying on a defence of mental impairment in the event that the jury finds he was the murderer.

The Crown, through prosecutor Mark Fernandez, outlined its case against Klobucar to the jury on Wednesday.

It is alleged Klobucar and his uncle went to buy marijuana from the deceased on December 30, 2013.

They both arrived in their own cars - Klobucar in a white Nissan Navara.

There were no drugs to buy, however, so the pair left in the uncle's car and drove elsewhere.

When they came back to pick up Klobucar's ute, the accused allegedly saw a blue Nissan Micra hatchback.

He asked his uncle whether it belonged to Mr Gajic, and allegedly said words to the effect of: "The blue Nissan Micra is a gay car, he must be a paedophile selling drugs and he might have a DVD relating to me from my past."

The Crown is arguing Klobucar had a tendency to be obsessed or preoccupied with people he thought were paedophiles, often believing men older than him were child sex offenders and acting violently or aggressively toward them.

That, the prosecution say, formed Klobucar's motive for the killing.

On New Year's Day, Klobucar returned to the Phillip address. Text message records suggest he had earlier asked his uncle for the address associated with the blue hatchback.

It is alleged he knocked, Mr Gajic answered, and Klobucar inflicted blunt force trauma to his head at the front door. There were no defensive wounds, the Crown say.

Klobucar is then alleged to have walked through the small one-bedroom unit, closed the back door, and dragged Mr Gajic's body back in to the hallway.

It is alleged he left detailed, blood-stained footprints in the hallway, which police later matched to a specific brand of Puma sport shoe.

Klobucar then took the dead man's phone, and records show the number was used to call his uncle and a former flatmate, two numbers that had never been dialled from Mr Gajic's mobile before.

The phone and an image of Klobucar's ute from a point-to-point speed camera were used to roughly trace Mr Klobucar's movements following the alleged killing.

It is alleged he went to his past housemate's place, and later his grandfather's place, where he drank and smoked cannabis.

Later that evening, he was arrested during a disturbance at the Lighthouse pub in Belconnen.

A shoe matching the footprint at the crime scene was seized from his white Nissan ute.

The court heard Klobucar had written a letter to a psychiatrist, detailing his version of events.

He said he had found Mr Gajic already dead when he arrived, and, panicking, took his phone intending to call police or go to the Belconnen police station.

He said he then decided not to alert authorities because he thought he was going to be falsely accused of the killing.

Klobucar said he believed his former flatmate had been responsible for Mr Gajic's death.

Mr Fernandez said the forensic evidence and the evidence of the Crown's witnesses did not support that version.

Klobucar's senior counsel John Purnell, SC, opened to the jury later on Wednesday.

He told them Klobucar denied causing Mr Gajic's death, meaning the prosecution must prove he did so, and that he had intended to kill or cause serious harm to the deceased.

The majority of his submission, however, focussed on the defence of mental impairment.

Mr Purnell told the jury three of Australia's best psychiatrists - including one first approached by the prosecution - had found Klobucar was mentally impaired at the time.

"You will hear three of the best forensic psychiatrists in Australia telling you that in their opinion, the accused has a defence of mental impairment," he said.

"Mr Klobucar says 'I didn't do it'. If you find that he did and he had the necessary intent, you then go on to decide whether he was mentally impaired."

The trial continues before Justice Hilary Penfold on Thursday.