Canada needs tougher legislation for people found not criminally responsible for their offences, the mother of a man beheaded on a Greyhound bus said Thursday.

Carol de Delley, shown in an interview with CBC News earlier this month, said her family is seeking justice for the death of her son, Tim McLean. (CBC)

Speaking during a vigil at the Manitoba legislature, Carol de Delley said federal law must be revised so that people with mental illness found guilty of a crime cannot eventually be released.

"In Canada, the question here becomes treatment or punishment," she told a crowd of about 150 people. "Why not both? I believe that treatment and punishment should go hand-in-hand."

Tim McLean, a 22-year-old carnival worker, was on his way home to Winnipeg from Alberta on July 30 when he was attacked as he slept on the bus.

The man accused of second-degree murder in McLean's killing, Vincent Li, was declared fit to stand trial earlier this month following an interim psychiatric assessment. He faces the possibility of an automatic life sentence with no chance of parole for at least 10 years.

'The general public is not necessarily very well informed about what happens when someone is found not criminally responsible'—Defence lawyer Sarah Inness

The outcome of Li's trial will largely depend on whether experts think he was criminally responsible for his actions, defence lawyers have said.

For an individual to be found not criminally responsible, they must have a mental disorder and that illness must be found to have had an impact on behaviour during the situation being examined.

A person found not criminally responsible is brought before the provincial mental-health review board annually for an assessment. If the individual can prove to the board that he or she is no longer a risk to society, there is a possibility of eventual release.

De Delley said Thursday she's worried that Li could eventually be released if the court finds him not criminally responsible for her son's death. She accused the current system of being more concerned for people accused of crimes than the victims.

"My son Timothy lost his life, and our lives will never be the same without him being a part of it," she said.

"Timothy's 23rd birthday would have been two weeks ago, on Oct. 3. We had his favourite foods for dinner and a cake with good friends. But Timothy wasn't there, and he never will be again."

Release not guaranteed: experts

Dressed in T-shirts designed with a photo of McLean's face, those who attended the vigil listened quietly as de Delley made her impassioned plea.

"Myself and my family are seeking justice for Timothy [and] mercy for ourselves."

Some experts said, however, that being found not criminally responsible for a crime in no way guarantees release, or even an easy ride.

"The general public is, on the whole, not necessarily very well informed about what happens when someone is found not criminally responsible," Winnipeg defence lawyer Sarah Inness said.

"They truly are not getting away with it. It's a humane way of dealing with people that are mentally ill. It's different than dealing with criminals."

Others have said that most people declared not criminally responsible spend much more time in psychiatric hospitals than they would if they were convicted of the same crime and given a sentence.

De Delley has previously said there should be no possibility — however remote — of future release if someone is found not criminally responsible.

Witnesses said the attack on McLean, which occurred as the bus was travelling in the vicinity of Portage la Prairie, Man., appeared to be unprovoked; a man sitting next to him simply stood up and started stabbing him, then cut up his body.

With files from the Canadian Press