TORONTO -- A lawyer for a Toronto senior charged with murder urged a jury to find the man not criminally responsible due to a mental disorder, arguing Tuesday that his client suffered from dementia which left him unable to appreciate the nature of his actions.

But a Crown prosecutor said Peter Brooks was not delusional, understood what he was doing, and lied about what he could and could not remember of what happened at a senior's residence in Toronto three years ago.

Brooks, 76, has pleaded not guilty to the first-degree murder of 72-year-old Jocelyn Dickson and the attempted murder of 91-year-old Lourdes Missier.

Brooks' lawyer told the jury in closing arguments that a large part of their task will be trying to understand the senior's state of mind on the date of the alleged offences.

"If you all conclude that there is a feather's weight more likelihood that Mr. Brooks was not criminally responsible due to a mental disorder, then your deliberations would have concluded," said Charn Gill.

"Peter is still unable to appreciate the moral wrongfullness of his actions...My submission to you is that his brain has been unfortunately ravaged of that ability and it will likely never return due to the degenerative nature of dementia."

The trial has heard that late one night in March 2013, Brooks used his cane to attack Dickson and Missier in their beds at the Wexford Residence in Toronto's east end.

Brooks has testified that a spirit in a dream told him to "beat the crap" out of his two fellow residents, and has insisted he didn't actually intend to harm anyone.

For a not criminally responsible finding, the defence must prove that Brooks suffered from a mental disorder on the day of the attacks and must prove that at the time of the offence, he was incapable of either appreciating the nature and quality of the act, or knowing that it was wrong.

Gill emphasized to the jury that two psychiatrists who testified at the trial found Brooks had dementia on the day of the attacks, although they disagreed on the severity of the condition with their diagnosis ranging from mild to moderate.

He also noted that the two psychiatrists agreed Brooks has damage to the frontal lobe of his brain -- which deals with impulse control, emotional regulation and perspective -- appeared disinhibited and did not appear to appreciate the seriousness of the proceedings against him.

"Both psychiatrists are opining that Peter had a neurocognitive disorder, or dementia, on day of offence," Gill said. "He was simply unable to appreciate the nature and quality of his actions."

The trial has heard Brooks had testy relations with both women who were attacked, who he described as "annoying" and aggravating him constantly.

Brooks testified the women were united in an alleged attempt to have him move out of the facility.

Gill argued that Brooks had formed a long-standing belief that residents at the nursing home were "conspiring against him."

"Peter feels justified in his actions because of his belief," Gill said. "Peter has been applying his own moral code for quite some time."

But the Crown argued that Brooks understood of the nature of his acts and knew what he was doing was wrong.

"Don't be fooled by him," prosecutor Donna Kellway told the jury. "Mr. Brooks' claim to not remember or his claim to not be conscious of what he did is simply lies."

Kellway noted that Dickson told a staff member before she was attacked that Brooks had threatened to kill her.

"She ended up dead, killed by Peter Brooks, because he disliked her and wanted her dead," Kellway said.

"I urge you to find that he knew what he was doing was not only legally wrong but that it was morally wrong."

The jury has heard that Brooks allegedly attacked Missier first, swinging his cane at the head of the woman who was awake at the time and raised her hands to protect herself. She was left with fractured fingers, bruises and lacerations on her face.

While staff were responding to what had happened to Missier, Brooks quietly made his way to another floor, where Dickson, a woman who was paralysed on one side of her body, was asleep in her bed, the Crown said.

Using his cane once more, Brooks delivered at least seven blows to Dickson's head causing "massive" injuries that led to the woman's death, the Crown has said.