Luka Magnotta has admitted killing Concordia University student Jun Lin in May 2012, but his defence lawyers will spend the next few weeks attempting to convince a jury that he is not criminally responsible for that act because he was mentally ill at the time.

"I intend to show you that at the time of the events, he was not criminally responsible," Magnotta's defence lawyer Luc Leclair told a Montreal courtroom on Monday, the first day of the highly anticipated trial.

Magnotta has pleaded not guilty to all five charges against him, including first-degree murder, but has agreed to the underlying facts of the case.

In the lead-up to the trial, it was speculated Magnotta's lawyers would rely on the not criminally responsible, or NCR, defence. Quebec Superior Court Judge Guy Cournoyer confirmed that soon after the trial began when he told the jury their task would be "to determine whether he committed the five offences with the required state of mind for each offence."

Under Section 16 (1) of Canada's Criminal Code, a person cannot be found criminally responsible "for an act committed or an omission made while suffering from a mental disorder that rendered the person incapable of appreciating the nature and quality of the act or omission or of knowing it was wrong."

The important piece is not simply the mental disorder. The important piece is the next part of the phrase, which says mental disorder which renders the accused incapable. - Patrick Baillie, forensic psychologist, lawyer

It's the second part of that statement that is key to a successful NCR defence, says Patrick Baillie, a Calgary-based lawyer and forensic psychologist with Alberta Health Services who has provided expert testimony in several NCR cases.

"The important piece is not simply the mental disorder. The important piece is the next part of the phrase, which says mental disorder which renders the accused incapable," Baillie said.

"This isn't something that distorted the person’s thinking or they were depressed and they did something they wouldn't ordinarily do because they just didn't care, but rather the mental disorder left them incapable of knowing what they were doing or knowing that it was wrong."

Premeditated behaviour doesn't rule out NCR

NCR cases begin with a presumption of sanity, and it's up to whoever raises the NCR application — in Magnotta's case, the defence — to overcome that presumption and prove the opposite.

"It's done on a balance of probability. It doesn't have to be beyond a reasonable doubt," said Baillie.

In Magnotta's case, the Crown will likely attempt to convince the jury that the behaviour Magnotta exhibited prior to and after the alleged crime shows that he clearly knew what he was doing. Crown prosecutor Louis Bouthillier​ told the jury Monday that he intends to prove Magnotta planned his crime six months in advance.

Andrea Yates, seen talking with her lawyer in 2002, drowned her five children in a bathtub in Texas in 2001 but was found not guilty by reason of insanity. (Pool/Reuters) But, Baillie says, that doesn't necessarily invalidate an NCR defence, because a defence lawyer only has to prove one of the two conditions: that the accused was incapable either of knowing what they were doing or of knowing it was wrong.

"You can still have planned and premeditated behaviour; the issue is whether or not you knew that it was wrong," he said.

A typical example is the case of Andrea Yates, the Texas mother of five who drowned her children but was found not guilty by reason of insanity in 2006.

"She knew what she was doing but did not believe that it was wrong because in her distorted way of thinking, she came to the conclusion that the children would be better off dead," Baillie said.

'Organized behaviour' could undermine defence

In Magnotta's case, the defence might have difficulty proving that he "didn't appreciate the nature and quality of the acts," said Michael Lacy, a veteran criminal trial lawyer and a partner at Greenspan Partners LLP in Toronto.

The Crown will likely argue that Magnotta acted with "orderly conduct," he said. The actions Magnotta is accused of — which include killing Lin, committing an indignity to his body, publishing and mailing obscene material and harassing Prime Minister Stephen Harper and other MPs — are inconsistent with someone suffering from a serious mental disorder, Lacy said.

The Crown will also likely bring up the allegation that Magnotta travelled to Europe immediately after the alleged crime, sparking a massive international police manhunt.

"That also evidences a level of organized behaviour that, at least viscerally, for most lay people... is sort of inconsistent with someone having a serious mental disorder," Lacy said.

However, mental illness is complicated, and the behaviour could be a part of whatever disorder Magnotta may have, Lacy said.

Defence to argue Magnotta was schizophrenic

The defence said in court Monday that it will show that Magnotta was schizophrenic and was hearing voices and feeling persecuted before the crime occurred. Schizophrenia is one of the most common diagnoses in NCR cases, Baillie said.

"Most NCR cases come back to some form of delusion or hallucination and therefore, typically, are tied to psychotic disorders: schizophrenia, a reaction to bad drugs, postpartum depression, those kinds of conditions," he said.

Lacy also stressed the fact that while Magnotta has not been diagnosed as a psychopath, psychopathic behaviour itself does not render an accused not criminally responsible.

"Having no conscience about what you do is not a disease of the mind that would render you not criminally responsible," he said.

Vince Li was found not criminally responsible in 2009 for beheading a fellow passenger on a Greyhound bus. Psychiatrists testified Li was schizophrenic and suffering a major psychotic episode at the time of the gruesome killing. (John Woods/Canadian Press) The challenge for psychiatric experts testifying in NCR cases, Baillie said, is assessing after the fact how an individual behaved in the moment. This is where a past medical record becomes key.

"In the majority of NCR cases, there is a full medical record," Baillie said. "These people are not strangers to the mental health system. A majority of them have had recent hospitalization, so there is a psychiatric record."

That is, in fact, what happened in Magnotta's case, according to Leclair. The defence attorney told jurors Monday that his client had had a one-hour psychiatric assessment at a Montreal hospital just weeks before Lin was killed. He also said the court would hear from health workers who treated Magnotta and be presented with his past medical record.

Yates, too, had had contact with mental health professionals, as did Vincent Li, who was found not criminally responsible in the 2008 death and beheading of fellow Greyhound bus passenger Tim McLean. Yates had been diagnosed with postpartum psychosis and depression and had attempted suicide; Li had been previously hospitalized for delusional behaviour.

NCR cases rare and not usually violent

When there isn't a mental history, "there has to be very thorough evaluation to make sure the person is not malingering, or more commonly known as faking it," Baillie said.

Baillie said he expects multiple psychiatrists to testify at Magnotta's trial but noted that in his 20 years as an expert on NCR cases in Calgary, the successful ones have been those where the Crown's experts and the defence experts agree.

"Typically, when you get into the battle of the experts, at least out here, the court comes to the conclusion that the accused was not NCR," he said.

NCR cases are rare — and do not usually involve violent offences, Baillie said. Much more common are cases in which a person commits an impulsive act like a theft or leaving a restaurant without paying because of some delusional belief or perceived danger ("the restaurant diners have turned into zombies and are out to hurt me," for example).

"Just under 10 per cent of [NCR] cases are serious personal violence, and the majority are violence against family members and friends," Baillie said.