Vincent Li, seen in this courtroom sketch, listens to proceedings during his second-degree murder trial in Winnipeg on Tuesday. (Tom Andrich/Canadian Press)

Believing he was acting on orders from God, Vincent Li attacked a stranger on a Greyhound bus last summer, mutilating his victim before decapitating him and cannibalizing part of the body.

In a clear voice, Li pleaded not guilty Tuesday in Winnipeg to a charge of second-degree murder in the death of 22-year-old Timothy McLean of Winnipeg on a Greyhound bus in Manitoba last July.

The Crown and defence have agreed on a statement of facts read to the court on Tuesday that suggests Li was mentally ill at the time of the slaying.

And forensic psychiatrist Dr. Stanley Yaren gave evidence that Li was diagnosed as schizophrenic and suffering a major psychotic episode — tormented by auditory hallucinations — at the time of the killing.

Yaren, head of Manitoba's forensic psychiatry program, said Li was delusional, believing God had told him to board the bus carrying a concealed knife. Yaren said it appeared Li sat beside McLean only because the young man offered him a friendly greeting.

The psychiatrist testified that according to Li, God told him that McLean was a "force of evil" who was about to stab Li unless he protected himself.

Even after the killing, Li believed McLean had supernatural powers and would come back to life unless he dismembered the body and spread the body parts around the bus. Yaren testified Li was not capable of understanding his actions were wrong.

In the agreed statement of facts, the Crown and defence say that Li, 40, apologized to police when he was arrested and pleaded with officers to kill him. The statement says Li attacked McLean "for no apparent reason" and ignored other horrified passengers as he stabbed the young man.

Court was told Li wrote to his ex-wife before leaving Edmonton aboard the Greyhound.

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

Crown attorney Joyce Dalmyn said Li got on the bus in Edmonton and disembarked at a rural stop in Manitoba. He stayed there overnight, selling and burning most of his possessions. He was there 24 hours before getting on the bus again.

Victim trapped

After a stop between Brandon and Portage la Prairie, Man., Li moved to sit beside McLean at the back of the bus. McLean was listening to an iPod.

Family and friends of Tim McLean comfort each other outside the Law Courts in Winnipeg on Tuesday. (John Woods/Canadian Press)

Around midnight, near Portage la Prairie, Li started stabbing McLean.

McLean struggled to escape but, seated at the back of the bus, he had nowhere to go. The driver pulled over and horrified passengers fled for safety.

McLean's body was damaged in more than 100 places, the Crown said, noting the attack was so unrelentingly violent that some of the victim's body parts could not be found.

Court was told Li had no criminal record. He came to Canada as an immigrant from China in 2001, where he also had no criminal past. Li has a computer science degree and became a Canadian citizen in 2005. He moved from Winnipeg to Edmonton in 2006.

Li worked as a caretaker in a church, as a supervisor at a McDonald's restaurant and as a newspaper carrier. But, court heard, he was prone to unexplained absences from work and sometimes took long road trips on the bus while offering friends and family members long, rambling talks.

Despite the urging of those close to him, he refused to seek medical treatment.

RCMP officers said Li's responses were appropriate and polite when he was finally arrested.

Li asked police to kill him

He declined a lawyer at one point, and told the police: "I'm guilty. Please kill me."

The defence is arguing that Li is not criminally responsible for his actions due to mental illness.

The finding means Li could be sent to a provincial psychiatric facility rather than to prison. He would be placed under the authority of a provincial review board, which would judge whether he poses a risk to the public. The review board has the power to keep Li under its authority or, if he is no longer considered a risk, discharge him.

McLean's family has been lobbying for changes to the Criminal Code, pushing for victim-protection legislation they call "Tim's law."

The proposed legislation would prevent a person found not criminally responsible of a crime from being released into the community. It would mean that the most violent, unpredictable people who have committed a crime would face incarceration for life, with no possibility of release.

Tim McLean's mother, Carol deDelley, told CBC News prior to the trial that she does not want to see Li ever released from custody.