Dr. Jess Ghannam told a Toronto court on Monday that Raed Jaser did not have consistent radical Islamic ideology, but adopted a persona as a pious Muslim to trick members of that community.
Ghannam concluded that Jaser had no intention to hurt anyone and he was motivated by his drug addiction and by wanting to take care of his family.
He told the sentencing hearing that Jaser had a "debilitating" addiction and "would do anything to stay high."
Jaser and his co-accused, Chiheb Esseghaier, were found guilty in March of a terror-related conspiracy to commit murder, which carries a sentence of up to life in prison.
The 12-member jury also found the men guilty of six other terror-related charges between them.
Ghannam — a licensed psychologist and clinical professor of psychology at the University of California, San Francisco — said he conducted interviews with Jaser's parents, brother, wife and sister-in-law as part of a psychiatric assessment.
Ghannam has testified in other terrorism cases in the U.S. and specializes in working with refugee, displaced, and immigrant populations from the Middle East, North Africa and South Asia.
He described Jaser — a permanent resident of Palestinian descent — as a man who was "broken psychologically" but had a caring family and "a special love of cats."
"He would rather spend what limited money he had on cat food, and feeding local neighbourhood cats or even feral cats, rather than spend money on food for himself or his wife," he said.
"Rather than kill an ant or a spider in his apartment, (he) would capture it and release it into the wild."
He said that he believes Jaser had neither the intention nor intent to hurt anyone and that he is a good candidate for rehabilitation. He said he is not challenging the jury's decision.
Court heard Jaser, 37, was doing drugs heavily while in high school, where he developed a $2,000-per-week habit by 1998.
Ghannam said Jaser had a taste for a wide variety of drugs including acid, opium, hash and marijuana, saying he was drunk and high almost all the time and undertaking cons and schemes to make money while also working as a taxi and limousine driver.
He said when Jaser started growing a beard and attending a mosque in 2009, Jaser's parents never believed he was committed to Islam but was instead trying to garner favours and cash.
During cross-examination, Crown attorney Croft Michaelson challenged Ghannam's objectivity in the case, accusing him of jumping to conclusions about Jaser's drug addiction and of not taking thorough enough notes during his interviews with Jaser and his family.
He also questioned Ghannam's credentials and criticized him for giving counsel diagnostic tests to be completed by Jaser ahead of time and for failing to monitor him while he was completing them.
During the trial for Jaser and Esseghaier, which began in February, court heard that an undercover FBI agent gained the men's trust and surreptitiously recorded their conversations, which made up the bulk of the evidence in the case.
The two were recorded speaking about alleged terror plots they would conduct in retaliation for Canada's military actions in Muslim countries.
In one of the recordings, Jaser was heard saying "everyone is a target" and that he wanted "the whole country to burn."
Esseghaier, who is a Tunisian national, refused to participate in his trial because he wanted to be judged under the rules of the Qur'an.
Jaser's defence lawyer argued his client was only faking interest in a terror plot as part of an elaborate con to extract money from Esseghaier and the undercover agent.