Costa, 71, sat still and stared straight ahead, not appearing to react as the jury delivered its verdict in the lengthy murder trial in the ACT Supreme Court.

The property owner had invited neighbour Terrence Freebody and his wife over for drinks and to show them his Mugga Way home on July 22, 2012.

Mr Freebody's wife left, and sometime later Costa pushed her husband off his chair, stomped on his face and neck, and then stabbed him.

Costa then dialled triple-0, telling the call-taker he had been attacked in his own home. Mr Freebody could be heard moaning in the background.

Police arrived to find a disturbing scene.

Mr Freebody lay dead in the dining room and Costa sat on a toilet with a blood-stained knife nearby.

He had been drinking and exhibited bizarre behaviour and aggression in the presence of police.

The trial centred around the killer's mental state and his alleged dementia.

The defence, led by Remy van de Wiel, QC, argued dementia made him not guilty by way of mental impairment.

Crown prosecutor Shane Drumgold, however, argued Costa had faked his dementia symptoms in order to avoid prosecution.

During the 11-day trial, jurors heard from a number of experts who gave evidence that they believed Costa suffered from dementia.

But Mr Drumgold told jurors the diagnosis had been based on flawed tests and did not meet other medical requirements for dementia.

The prosecutor told jurors that Costa's behaviour could be explained by intoxication.

The issue of his self-induced state of drunkenness was also central to the trial and the jury's decision.

Mr Drumgold said that a man suffering such severe dementia could not have maintained Costa's active lifestyle, which included ski holidays, driving, and managing substantial real estate holdings.

But Mr van de Wiel said Costa's grip on that lifestyle had been slipping for months or years, citing evidence that the offender's finances were in chaos, food in his home were out-of-date, and he had attempted to attend non-existent business appointments.

He argued the prosecution case relied on a "conspiracy theory" that the experts had been wrong.

The jury retired to begin deliberations on Monday afternoon, and handed down the guilty verdict a day later.

Mr Freebody was well-known in the Canberra and Queanbeyan communities.

He was considered a local sporting legend in Queanbeyan, both in rugby league and cricket, and played for the Prime Minister's XI.

His experiences during World War II were said to have affected him deeply.

After his death, he was remembered as a "really good bloke", a superbly talented sportsman, and an intelligent businessman, who built his wealth through a series of car garages, bike shops, cinemas, and a number of car dealerships.

The sentencing of Costa will begin on July 22 before Chief Justice Helen Murrell.

An updated forensic mental health report has been ordered.