Luigi Costa, 71, allegedly knocked Terrence Freebody off a chair, stomped on his face and neck, and then fatally stabbed him.

On Monday, Costa went on trial in the ACT Supreme Court for the death in his Mugga Way home on July 22, 2012.

Costa has pleaded not guilty.

Juries are usually given the option of returning verdicts of either guilty or not guilty.

But the jury was told on Monday that they had a third option – to find Costa not guilty by way of mental impairment.

Costa's defence team says their client suffers from dementia and has no memory of the alleged murder.

But the Crown – led by Shane Drumgold – argues that Costa has faked his symptoms to escape prosecution for Mr Freebody's death.

The court was told that Costa had invited Mr Freebody and his wife over for a drink about 1pm to thank them for watching his house and to show them his renovated home.

The wife left about 5pm, allegedly due to Costa's foul language, but Mr Freebody stayed behind.

About 5.03pm, Costa left a message on a friend's phone telling him to call the police, the court was told. About 5.06pm, Costa called triple-zero to ask for assistance, claiming he had been attacked and the assailant was still in the house, and then hung up.

The emergency call operator reported hearing groaning in the background during the call, the court was told.

The operator called back, but Costa put him on hold for 13 seconds before becoming aggressive, saying "get f---ed" and "I'll kill you".

Police arrived about 5.25pm to find Mr Freebody dead in the dining room and Costa sitting on the toilet with a blood-stained knife nearby.

Mr Drumgold, in his opening submissions, said Costa had pushed Mr Freebody off the chair, causing him to breaking his back.

Mr Drumgold told jurors that the moaning they would hear in the background of the triple-zero call had been Mr Freebody after the fall.

The prosecution alleged that Costa needed to explain the fall, but had not formulated an excuse.

Costa then stomped on Mr Freebody's face and neck to silence him, before killing him with a knife in an attempt to cover his tracks, Mr Drumgold said.

The prosecution argued that Mr Costa did not suffer from dementia and had managed his home and finances, and travelled without problems leading up to July 2012.

Mr Drumgold alleged that Costa has deliberately feigned memory loss and flunked mental health tests in order to escape prosecution.

But defence barrister, Remy van de Wiel, QC, said his client's behaviour had been consistent with a man suffering dementia.

Mr van de Wiel told jurors that the defence would not allege that an unknown person killed Mr Freebody.

He explained that defence teams worked on orders from clients, and Costa's instructions had been that he did not recall the alleged offence.

The barrister said the Crown's allegations had been speculation not supported by fact and he urged jurors to examine the evidence carefully and keep open minds until the end of the trial.

The trial before Chief Justice Helen Murrell will continue on Tuesday.