"Something forced me to do it," Magnotta told Marie-Frederique Allard, who assessed him for criminal responsibility and gathered the version of events earlier this year. "It just gave me this weird energy."
Her report is central to the defence case in light of the various charges Magnotta faces related to Lin's slaying.
Magnotta, 32, has admitted to killing Lin in May 2012 but has pleaded not guilty to first-degree murder and four other charges by way of mental disorder.
Twenty months after the slaying, Magnotta began providing Allard with details of the events, telling her Lin replied to his advertisement on Craigslist seeking a bondage partner.
How they hooked up was an element police said they weren't able to determine.
Magnotta and Lin met in person briefly for the first time the night of the slaying near a subway station before walking several blocks to the accused's apartment.
Allard said Magnotta told them they drank wine and had sex. Magnotta was the first to be tied up, but complained Lin was going too fast and was hurting him, striking him about the head.
The jury then heard from Allard that Magnotta became very anxious and took some sleep medication, Temazempam, and that Lin also asked for some.
During a sex break, Magnotta thought he saw a black car parked outside his building and became paranoid about a government conspiracy involving Lin.
Allard said Magnotta began hearing voices telling him to tie up Lin and to "cut it" and to "do it, he's from the government."
Subsequently, Lin had his throat cut.
"Jun Lin was the unfortunate victim of a psychotic man," Allard said. "Mr. Magnotta killed who he thought was a government agent."
The psychiatrist testified that Magnotta said he published the "One Lunatic, One Ice Pick" video of Lin in an effort to shut out the voices. Before his slaying, Lin had reportedly consented to having their sexual acts filmed.
Six days earlier, Magnotta had a similar encounter with an unidentified man but, with Lin, "something just happened in my brain."
Once Lin's body was dismembered, Magnotta said he kept hearing voices telling him to "give it back to the government."
Various parts of Lin's body were then sent in packages to political offices in Ottawa.
Magnotta said he considered sending Lin's torso, but could not figure out how.
Asked why body parts were mailed to two Vancouver schools, Magnotta told Allard he'd discussed the western Canadian city with Lin.
He hardly slept after Lin's slaying but did remember eating a pizza at one point.
"It is very scary, how can I do that?" Magnotta told Allard, referring to the killing.
The obsession with government and spying was present when his schizophrenia was initially diagnosed. Allard said Magnotta told her that anti-psychotic medications helped ease the paranoia but that he didn't take them all the time.
The fear of being spied on led to frequent address changes, multiple online profiles, the use of a wig and various changes of cellphone numbers.
Allard said he changed his name to Magnotta in part out of that same fear but also because he wanted to get away from his family. He chose the surname either for a video game character or a popular winery in Ontario's Niagara region.
While Magnotta didn't think he was currently being spied on, "he's convinced it happened in his past," she said.
In earlier testimony, Allard said Magnotta told her that an email he sent a British tabloid foretelling the murder of a human being was meant only as a threat to a journalist who'd ambushed him.
The Crown contends the communication to the London Sun suggests Magnotta was planning a slaying up to six months in advance of Lin's killing.
But Allard said he told her he was only trying to intimidate reporter Alex West, who'd interviewed him at a London hotel a few days earlier about videos showing cats getting killed.
It was Magnotta himself who had published the videos, which angered animal-rights activists who were trying to track him down.
Allard said Magnotta has always maintained he didn't plan to kill anyone and that he believes writing the email in December 2011 was "stupid" and showed a lack of judgment.
She also told the jury Magnotta thought his tactic of sending the email had worked because no story was published following his encounter with West.
The forensic psychiatrist has said she believes Magnotta was suffering from schizophrenia and was in a psychosis when he killed Lin.
Also on Tuesday, Allard told the court that Magnotta admitted making up his affair with convicted killer Karla Homolka in 2007.
Allard said Magnotta was seeking notoriety, but primarily wanted people to believe he was the victim of harassment. The Homolka story spread quickly and Magnotta lost control.
"I am not proud of this, I feel ashamed," Allard quoted Magnotta as saying.
He also admitted hiding his schizophrenia symptoms from Montreal doctors a few weeks before Lin's killing because he was reportedly afraid that revealing them would prevent him from moving to France to start a new life. It was a plan he'd hatched a few weeks earlier, convinced the government was spying on him.
Allard noted Magnotta went to great lengths to hide his schizophrenia because of the reaction others had toward his father, who also had the mental disease.
"I don't want to be like that," he told her. "They were afraid of him."
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