Magnotta has pleaded not guilty to five charges, including first-degree murder, stemming from the May 2012 slaying and dismemberment of Jun Lin, 33.
Quebec Superior Court Justice Guy Cournoyer's detailed final charge also opened the door to a conviction on second-degree murder or manslaughter on the murder charge.
But if the jury finds Magnotta not criminally responsible, Cournoyer said the verdict must apply to all five charges. It's also the issue he suggested jurors tackle first when deliberations begin Tuesday.
In addition to premeditated murder, Magnotta is charged with criminally harassing Prime Minister Stephen Harper and other members of Parliament; mailing obscene and indecent material; committing an indignity to a body; and publishing obscene materials.
Psychiatrists for the defence testified throughout the trial Magnotta is schizophrenic, was psychotic the night of the slaying and was unable to tell right from wrong.
Crown prosecutor Louis Bouthillier countered there was no mental illness at play and that the crime was planned and deliberate. He has said Magnotta should be found guilty of first-degree murder and the four other charges.
Cournoyer told the jurors they would need to answer two questions for the mental disorder defence to be accepted. Firstly, is it more likely than not Magnotta was suffering from a mental disorder at the time of the offence? And secondly, did the disorder make him incapable of knowing the acts were wrong?
He walked them through the various expert reports, medical documents and testimony they could consider. However, if the answer to either of those questions is no, then Magnotta is not exempt from criminal responsibility, Cournoyer said.
If the jury opts for a verdict of not criminally responsible, Cournoyer told them Magnotta would not be set free if he's considered a significant threat to public safety.
"Paranoid schizophrenia is a disease of the mind but it is for you to decide whether Mr. Magnotta was suffering form paranoid schizophrenia at the time of killing," he said.
"Under our law, the verdict of not criminally responsible by reasons of mental disorder is not a loose term, quite the contrary. There are specific criteria to determine whether the defence of mental disorder is applicable."
The jury will be required to render a unanimous verdict on each count.
Magnotta has already admitted to the physical acts of the case, meaning the jury will need to determine intent as well as the level of the planning in the slaying.
Cournoyer told them they must rely solely on the evidence they have heard over 40 days since the trial began in late September.
Magnotta did not testify and didn't meet with a Crown psychiatrist, but the judge said he was not obliged to do so as part of his mental disorder defence.
"You must not infer Mr. Magnotta's guilt from his failure to testify .... you cannot use his silence at trial as evidence of his guilt," the judge advised.
Among the evidence to consider, Cournoyer said the jury will have to keep in mind that Magnotta's statements given to defence psychiatrists about facts surrounding the events in May 2012 and his own state of mind have not been independently proven.
The Crown must prove its case beyond a reasonable doubt, but a mental disorder defence puts the onus on Magnotta and lawyer Luc Leclair to prove the accused has such a disorder and that he did not know what he was doing was wrong.
"Mr. Magnotta must prove that it is more likely than not that he suffered from a mental disorder to such an extent at the time the offences were committed that he is not criminally responsible," Cournoyer told the jury. "This is a lower standard than proof beyond a reasonable doubt."
He said they must not be influenced by public opinion and must assess the evidence impartially and without "sympathy, prejudice or fear."
Fourteen jurors heard the evidence but two male jurors were sent home once the judge's instructions were complete. One of them had a trip and was excused while the other was chosen at random.
Eight women and four men will ultimately decide Magnotta's fate.
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