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The man who murdered six Muslim men in a Quebec City mosque in January 2017 had been suffering with mental illness for years and wanted to kill, a psychologist who evaluated the gunman said in court Monday.

Alexandre Bissonnette initially wanted to shoot people in a shopping centre but decided against that attack and turned his attention to the mosque, Marc-Andre Lamontagne said during sentencing arguments.

Lamontagne, an expert in evaluating people's level of dangerousness and their risk of repeat offending, met Bissonnette twice in early April for a total of seven hours, at the request of the killer's defence team.

Bissonnette, 28, pleaded guilty in March to six charges of first-degree murder and six of attempted murder related to the deadly mosque shooting.

His lawyers began presenting their case Monday in order to obtain the shortest possible sentence for their client.

Lamontagne said Bissonnette harboured violent and hostile thoughts for years — sentiments that were initially directed toward his peers at school who bullied him, as well as toward former teachers.

His hostility eventually became more generalized, Lamontagne said.

Over the last few years, Bissonnette came close to killing himself numerous times, Lamontagne said. He would write goodbye letters and put the barrel of his gun in his mouth.

The killer also lied about his past psychological problems in order to obtain a gun permit, Lamontagne said.

Lamontagne said Bissonnette told him that six weeks before the mosque shooting, he considered murdering people in a Quebec City shopping centre.

He travelled with his guns as far as the mall parking lot before abandoning the idea.

Bissonnette didn't want to kill just anyone, said Lamontagne. He said Bissonnette chose the mosque because "he convinced himself that if there was at least one religious extremist inside," it would be worth it to murder people because it would save lives.

The idea was to seek vengeance from those who had persecuted him, but also to commit a "grandiose act" so no one would laugh at him after he was dead, Lamontagne added.

Bissonnette is neither a psychopath nor anti-social and did not hold delirious ideas or suffer from psychotic symptoms, he said.

Defence lawyer Charles-Olivier Gosselin asked Lamontagne if he thought Bissonnette could be rehabilitated.

"There are reasons to believe that he can," the psychologist responded.

Earlier on Monday, the defence presented its first witness, a former high school teacher who said Bissonnette was bullied and intimidated mercilessly in school.

Students would laugh at him, hit him and throw him against the wall regularly, said Lucie Cote.

Cote, who was Bissonnette's teacher in two different years at two different high schools, said he developed reflexes of nervousness and fear and did not defend himself.

She told Superior Court Justice Francois Huot she came to testify on her own accord and that her presence wasn't requested by the killer's defence team.

"After everything he endured, I couldn't stay silent," she said.

Cote said she never witnessed a worse case of intimidation and bullying toward a student in her 32-year career.

She said she took about 20 minutes of class time one day to scold her students about the way they treated him. Things changed a bit, but it didn't last, she said.

Cote said she cried when she heard the news of Bissonnette's arrest.

He wasn't a monster, said the retired 71-year-old.

Cote asked the judge to "give him hope."

Bissonnette's first-degree murder conviction carries an automatic life sentence with no chance of parole for 25 years.

But he can also receive consecutive sentences, which means he could spend up to 150 years in prison.

Bissonnette's lawyers have claimed in court that giving their client a 150-year jail term would be equivalent to sentencing him to death by incarceration.