Defence witness Joel Watts testified that an exhaustive look at the evidence and Magnotta's medical history led him to believe that while the accused was aware of his actions, he could not differentiate right from wrong.
Magnotta has admitted to killing Lin in May 2012, but has pleaded not guilty by way of mental disorder.
The Crown contends the slaying was planned and deliberate.
Watts' assessment is similar to that of another psychiatrist hired by the defence. Marie-Frederique Allard testified previously that Magnotta's schizophrenia was out of control in May 2012.
Watts met with Magnotta face-to-face over 38 hours between September 2012 and September 2013. He spoke to relatives and one friend and also saw Magnotta for a few hours last month.
"My opinion regarding Mr. Magnotta's mental state at the time ... is that he was suffering from an acute episode of psychosis," Watts said.
"He suffers from schizophrenia and on those days (in May 2012), he was suffering from an acute psychotic break of that schizophrenia."
Watts said the psychosis is linked to each of the five charges against Magnotta: first-degree murder; 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.
"Despite Mr. Magnotta's psychotic episode on those dates, my opinion was he was able to appreciate physically what he was doing and the consequences of what he was doing," Watts added.
"For each of his charges, I do not think he knew it was wrong, due to his psychosis."
That wasn't his initial impression.
Watts first met Magnotta when he travelled with Montreal police detectives to Berlin for his arrest in June 2012.
He was on the plane because German authorities decreed that a psychiatrist had to accompany Magnotta home and keep an eye on him.
Watts met with Magnotta briefly in Berlin and felt he was putting on a "show." He said the accused's expression was "flat" and his attitude "child-like" and "whiny."
He testified that Magnotta's responses had a dramatic quality.
Watts clarified in his testimony Friday that his initial impression of Magnotta was an observation, not a diagnosis.
It was also coloured by police comments made on the way to Germany that Magnotta might be faking his mental health symptoms.
"My role was not to make a diagnosis (in June 2012) and I didn't make a specific diagnosis at that time," he said.
Watts was later hired by the defence to conduct an assessment of Magnotta's criminal responsibility. Armed with his medical history and a fuller picture, the psychiatrist said he has no doubt he was suffering from psychosis.
He signed off on his final 124-page report last February and called the evaluation "an arduous and difficult process, possibly the most difficult case I've done yet."
Watts spoke to Magnotta's mother, maternal grandmother, sister and father.
His younger sister, Melissa, told him her brother had become increasingly distant, vacant and unable to communicate, as of 2006.
Watts quoted her as saying that he "looked dazed" during a Skype chat in March 2012 and that "he wasn't Eric," referring to Magnotta's birth name.
"I thought he was possessed," she told Watts. "He had a blank look, what he was saying was not making sense."
She also told Watts about an email she believes was sent by Magnotta three days before his arrest. The doctor said it contained the following message: "How does it feel to be the sister of a murderer?"
A copy of the email was never recovered.
The psychiatrist returns to the stand on Monday.
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