Dr. Joel Watts has testified he believes Magnotta was suffering from a schizophrenia-linked psychotic episode when he killed and dismembered Chinese student Jun Lin in May 2012.
The Crown is attacking the psychiatrist's credibility because that conclusion differs from his notes while in the police department's employ, which suggest he agreed with detectives that Magnotta was acting.
Watts wrote in those notes that his initial impression was similar to a police hypothesis that Magnotta was putting on a "show" following his arrest in Berlin in June 2012.
But in a 124-page report he prepared for the defence, Watts concluded Magnotta was suffering from a psychotic episode.
Watts said his police role ended when Magnotta landed in Montreal and that his views in Berlin did not constitute a diagnosis.
"I wouldn't characterize it as changing sides, I was paid by the police to accompany someone who was being extradited," said Watts, qualifying his role with Magnotta on the flight home as a "limited treating role" before the defence came calling.
Magnotta has pleaded not guilty to the slaying and dismemberment of Jun Lin in May 2012.
The accused admits having caused Lin's death, but Watts and another psychiatrist have testified he was suffering from psychosis and schizophrenia and was incapable of telling right from wrong the night of the killing.
The Montreal-based psychiatrist said he consulted colleagues before accepting the defence's offer. He noted it was a precedent-setting situation to have a psychiatrist involved in an extradition.
Watts had just a few years under his belt in 2012 and said he felt honoured to travel to Germany, calling it a professionally exciting experience.
"The sense that I got from the police and the other individuals on the airplane was this was a very unusual, uncommon and unheard of sort of exercise," Watts said.
He agreed with the Crown that there was no evidence of Magnotta showing any apparent outward signs of psychosis during the roughly two weeks between Lin's slaying and the assessment by a Berlin prison psychiatrist that he was in a psychotic state.
But Watts warned that Magnotta's case history didn't indicate outward signs of psychosis.
"You have to be careful there because it sounds as though you are characterizing it a like a light switch that goes on and off," Watts warned.
Earlier on Monday, the jury heard that Magnotta told Watts he didn't know why he was wearing Lin's clothing after the Chinese student's killing.
Surveillance video images at Magnotta's apartment captured the accused emptying the contents of his apartment while wearing Lin's baseball cap and yellow T-shirt. The cap was subsequently recovered in Berlin when Magnotta was arrested.
"I asked Mr. Magnotta about that and he described to me that he couldn't remember why he decided to wear the clothing other than the fact that he liked the clothing Mr. Lin had," Watts said. "There was a ball cap that he particularly liked. He said it looked nice."
Defence attorney Luc Leclair also asked Watts about "Manny," a man Magnotta has cast as an abusive tormentor who allegedly forced him to stop taking his anti-psychotic medications and to film videos in which cats were killed.
Manny has been a central figure in Magnotta's interviews with psychiatrists, but the existence of the American from New Mexico has never been established.
Watts believes the man does exist.
"But I think at some point Mr. Magnotta's experiences of Manny ceased to be based in reality and his experiences were actually hallucinatory," he said.
In addition to first-degree murder, Magnotta is also 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.
The Crown has argued the crimes were planned and deliberate.
Quebec Superior Court Justice Guy Cournoyer told jurors Monday he wants to deliver his final instructions the first week of December and that they should be deliberating by the end of that week.
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