WINNIPEG – WARNING GRAPHIC CONTENT: A psychiatrist says a man who stabbed and beheaded a fellow passenger on a Greyhound bus believed the voice of God was telling him to do it.

The voice told Vince Li to get on the bus and sit next to Tim McLean, Dr. Stanley Yaren told Li's second-degree murder trial Tuesday.

"A voice from God told him Mr. McLean was a force of evil and was about to execute him," Yaren, a witness for the Crown, told the judge hearing the case.

Li, 40, believed he had to act quickly to protect himself, Yaren said.

"In response to that, in a state of panic and fearful for his life, he carried out the acts that he did."

But that wasn't enough.

Li , whom Yaren diagnosed as schizophrenic, believed the 22-year-old McLean was still capable of coming back to life, so he continued to mutilate the body and scattered the parts around the bus, the psychiatrist testified.

Although he admitted his guilt to officers that night last July, Li pleaded not guilty on Tuesday. His lawyers are arguing he is not criminally responsible because he is mentally ill.

Yaren concurred.

"Mr. Li did not understand he was killing an innocent bystander. He did not understand his actions were wrong."

An agreed statement of facts read out in a Winnipeg courtroom said Li, blood still smeared on his face from the attack, politely apologized to police when he was arrested and pleaded with officers to take his life.

"I'm sorry. I'm guilty. Please kill me."

Yaren said Li is still psychotic and believes it's just a matter of time before God kills him. He continues to have hallucinations and hear voices, but is on strong anti-psychotic medication.

Li is as much a victim as McLean, Yaren suggested.

"It would be in some sense easier if Mr. Li was an anti-social psychopath with a history of malicious behaviour, but he isn't that. He is, as I've come to know him, a decent person. He is as much a victim of this horrendous illness ... as Mr. McLean was a victim."

The statement said Li attacked McLean "for no apparent reason" and ignored other horrified passengers as he repeatedly stabbed the young man, who unsuccessfully fought for his life.

"Tim McLean struggled and tried to escape," Crown prosecutor Joyce Dalmyn said.

But McLean couldn't get away because Li was blocking the aisle.

When the bus pulled over near Portage la Prairie, Man., Li was engrossed with stabbing and mutilating McLean's body. Passengers fled the bus and stood outside.

It was then that Li tried numerous times to leave the bus. But he was locked inside and, according to the statement, returned to McLean's body and methodically carved it up further. Police arriving on scene asked him to drop the knife and he said he "had to stay on this bus forever."

But he eventually tried to escape out a window and was taken into custody.

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Police said McLean's body parts were found throughout the bus in plastic bags, although part of his heart and both eyes were never found and were presumed eaten by Li. He has denied that, but "there is no other possible location for those items," said Dalmyn.

The victim's ear, nose and tongue were found in Li's pocket.

McLean, a carnival worker, had been returning home to Manitoba after working at a fair in Alberta. Passengers have said he was sleeping near the back of the bus and listening to music on his earphones when he was attacked.

No one who witnessed the horror was expected to testify.

McLean's family and friends, many wearing T-shirts with his picture on them, wept as the grisly details were read out in court.

His mother, Carol deDelley, has said she wants the law changed so anyone found not criminally responsible for a crime still serves time behind bars. But legal experts say the defence is rarely used and doesn't mean the criminal walks away scot-free.

The agreed facts also presented some of Li's background. He was born in China in 1968 and came to Canada in 2001. He became a citizen in 2005. He graduated from a business college, but never got a job in his field.

He didn't have many friends and was divorced in 2006. Li had ``mental problems," according to those who knew him, but they had not known him to be violent

Yaren said Li was briefly hospitalized in 2003 or 2004 after he was picked up by Ontario Provincial Police, who found him walking along a highway "following the sun" as ordered to by God.

Li's former wife said he used to be gone for long periods of time, took unexplained bus trips and sometimes rambled. He was hospitalized briefly but never sought medical attention.

The statement outlines how Li got off the bus in Erickson, Man., where he spent the night on a park bench before boarding another bus July 30. It was as that bus neared Portage la Prairie that he moved to the back where McLean was sitting.

Before he left on his trip, court heard how he left his wife a note.

"I'm gone. Don't look for me. I wish you were happy."