Should one of Canada's most notorious serial killers be granted a new trial because of an alleged error by the judge?

The Supreme Court of Canada will consider that question Thursday when lawyers for Robert Pickton argue that his convictions for killing six Vancouver prostitutes should be overturned.

Pickton's lawyers say B.C. Supreme Court Justice James Williams' jury instruction undermined the defence at his 2007 trial, which ended with six convictions for second-degree murder. Pickton, a Vancouver-area pig farmer, was sentenced to life with no parole eligibility for 25 years - the maximum penalty.

The appeal is focused on a narrow point of law, but one that is often the source of successful appeals: whether a trial judge erred in instructing the jury.

It turns on a question that jurors posed to Williams on the sixth day of their deliberations: could they convict Pickton if he was only an accomplice in the killings?

Williams had crafted his initial charge to the jury after consulting with the prosecution and defence, and told the jury it had to render a guilty verdict if it found Pickton was the actual killer. After their question, Williams revised his instructions to the jurors, telling them they could still convict if they found Pickton was the killer or an “active participant” in the killings.

Pickton's lawyers say that confused the jury. They argue that the Crown, at trial, rejected suggestions that Pickton might have had an accomplice.

“Put simply, the jury's critical question was incorrectly answered, the 'goal posts' were changed by the amendment at a very late and impossible stage of the trial, and the Crown gained a significant, unjustified advantage,” say Pickton's lawyers, adding that his conviction “might even fall outside the ambit of criminal liability.”

However, Crown lawyers argue that the case against Pickton was so overwhelmingly strong that he would be reconvicted if a new trial were ordered. Therefore, they argue, his appeal should be rejected.

“He is a self-confessed killer. He admitted he was caught because he became sloppy at the end,” states the Crown brief.

“Ordering a new trial in this case, where conviction is inevitable, would serve only to 'detract from society's perception of trial fairness and the proper administration of justice.”'

The Crown argues that the defence itself led evidence of third-party involvement in the killings.

“In fact, much of it came of out of Pickton's own mouth. He said he was the 'head honcho' and that others were 'involved too.”'

Pickton was originally charged with 26 counts of first-degree murder but the judge severed 20 cases from his trial. The B.C. Attorney General has said if Pickton's appeal is successful it will retry him on all 26 counts.

As is its custom, the Supreme Court is expected to reserve judgment Thursday.