Three of the most eminent psychiatrists in Australasia have clashed over Chris Simpson's state of mind when he killed his mother.
Two of them say he was insane, but the third says he knew what he was doing was wrong.
Dr David Chaplow, New Zealand's director of mental health, and Dr Paul Mullen, professor of forensic psychiatry at Monash University in Melbourne, say that Simpson did not know what he was doing at the time was morally wrong.
But Professor Graham Mellsop, dean of psychiatry at Middlemore, says that Simpson understood the moral significance of his actions.
All three agree that Simpson was suffering from bi-polar illness - what used to be known as manic depression.
Simpson, aged 55, of Three Kings, who ran a men's health clinic in Remuera, is accused of the murder of his 82-year-old terminally ill mother.
Florence Marjorie Simpson was found lying partly out her hed at her Howick home on October 3 last year, the strap of a morphine bag wrapped tightly around her neck.
The Crown says it was a merciless killing.
But in his opening address, defence lawyer Paul Davison, QC, asked why a loving son would set aside all love, affection and concern and suddenly decide to kill his mother.
Mr Davison dismissed the notion of euthanasia which, he said, Simpson strongly opposed.
He also pointed out that, as a doctor, Simpson had the means to achieve his mother's death in a way that would neither cause controversy nor disclosure.
"Whatever it was that caused this otherwise loving son to have acted in the bizarre manner he did must have been a powerful and potent type of force"
The real explanation lay in Simpson's "tragically disordered mental state" which rendered him unbalanced and unable to appreciate that what he was doing was morally wrong.
It was a state of mind that caused an "eruption or outburst of manic behaviour" directed at his mother lying in a pitiful state and appealing for relief.
Mr Davison said that there had been a previous episodes of mania and depression in Simpson's life but the disorder began building again in April last year when he opened his men's clinic.
In asking for a verdict of not guilty by reason of insanity, Mr Davison said that there were "legal consequences" and it was not a "walk out the door result".
A former girlfriend, Marie Barrett, told the court that Simpson was stressed out following the opening of his clinic and by September he was "over the top".
Dr Chaplow told the court that Simpson told him he killed his mother by straddling her and putting a pillow over her head.
Asked why, Simpson told the psychiatrist: "I will never understand it as long as I live. I was drunk."
Simpson said he noticed that this left his mother with petecchial haemorrhages on her skin, which other doctors might recognise as a sign of asphyxia.
He said that, believing she was dead, he twisted the morphine tube around her neck and lifted her out of bed.
"In my disordered state of mind I wanted it to look like she had strangled herself and fallen out of bed," Simpson said.
Dr Chaplow, a defence witness, said that on the balance of probabilities, Simpson did not know his behaviour was morally wrong when he injected his mother with drugs and smothered her.
But by the time it came to putting the ligature round her neck, Simpson had said that any doctor would recognise what happened.
It was likely he had some moral awareness at that stage, although he was clearly impaired and clumsy in his attempted cover-up.
"What strikes you when you come to write the report and formulate your opinion is the bizarreness of a person who is a doctor with the skills of a doctor who then terminated his mother's life - one he loves - in such a bizarre fashion rather than await the days or hours of her natural demise, or taking a far more subtle route to bring about her end," Dr Chaplow said.
Excerpts of conclusions from Professor Mellsop, who will give rebuttal evidence for the Crown, were read in court.
Professor Mellsop said Simpson's alcohol dependence made conclusions difficult because the effects of alcohol mimic the bi-polar disorder factors and invalidate it.
He also reached the conclusion that because Simpson, while holding the pillow, was telling his brother to shut the door and keep his brother's partner away, he understood the moral significance of how others might see his actions.
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