A man accused of beheading and cannibalising a fellow Greyhound bus passenger is obviously insane and not criminally responsible, the Canadian government prosecutor told a judge yesterday.

The judge will decide today whether Chinese immigrant Vince Li should be held accountable for stabbing Tim McLean dozens of times last July and dismembering his body while horrified passengers fled.

Both the prosecution and the defence have argued Li can not be held responsible because he is mentally ill.

Li is accused of second-degree murder in the death of Mr McLean, a 22-year-old carnival worker. He has admitted he killed Mr McLean but pleaded not guilty by reason of insanity. Last year, Li was judged fit to stand trial.

But prosecutor Joyce Dalmyn said the prosecution had little choice but argue he not be held criminally accountable, after their own psychiatrists found him unable to "differentiate between wrong and right".

"He was out of touch with reality," Ms Dalmyn said, adding the court did not even need to hear from psychiatrists. "He believed God was telling him to kill the evil force beside him."

Ms Dalmyn said Mr McLean suffered a horrendous death, and his family wants to see Li put in prison, but the horrible nature of the crime does not allow the prosecution to abandon its duty to the court.

Witnesses said Li attacked McLean unprovoked, stabbing him dozens of times, as their bus travelled at night along a desolate stretch of the Trans-Canada Highway.

An agreed statement of facts between the prosecution and defence detailed how passengers stood outside the bus as Li stabbed and mutilated McLean’s body. Finding himself locked inside, he finally escaped through a window and was arrested.

Li apologised and pleaded with police to kill him.

Police said Mr McLean’s body parts were found throughout the bus in plastic bags, and the victim’s ear, nose and tongue were found in Li’s pocket.

A psychiatrist called by the defence said Li suffers from schizophrenia, and a psychiatrist called by the prosecution testified that Li cut up Mr McLean’s body because he believed that McLean would come back to life and take revenge.

Mr McLean’s mother, Carol deDelley, said she wants the law changed so anyone found not criminally responsible for a crime still serves prison time.

"It strengthens my resolve to change the laws that are currently in place because to me that are grossly inadequate in circumstances like this," she said outside court.