OTTAWA -- Eighteen years after a Halifax-area man was gunned down for having an affair with an outlaw biker's girlfriend, the Supreme Court of Canada has agreed to hear an appeal in the case against the alleged triggerman.

Sean Simmons, 31, was shot in the head in the lobby of a Halifax-area apartment building in October 2000.

Dean Kelsie was convicted of first-degree murder and conspiracy to commit murder in 2003 and sentenced to life in prison with no chance of parole for at least 25 years.

But the Appeal Court of Nova Scotia ordered a new trial after ruling last year that the trial judge erred in his instructions to the jury, particularly when it came to what the jurors could make of the hearsay evidence of co-conspirators.

The appeal ruling also said the trial judge should have mentioned manslaughter to the jury as an alternative verdict, even though Kelsie's lawyer didn't object that it wasn't.

In a decision issued Thursday, the Supreme Court of Canada agreed to hear the Crown's appeal of that ruling.

"The application for leave to appeal is granted," the court said.

Kelsie was one of four men who have been convicted of the crime, although only two of those convictions stand; testimony at the trials indicated that it was Kelsie who pulled the trigger, which he denied.

In the Nova Scotia appeal court ruling, Justice David Farrar set out how Simmons came to be targeted.

"Mr. Simmons, in the early 1990s, had been closely affiliated with the Halifax Hells Angels and hoped to become a member. By 1993, however, he was targeted for violence by the club and was beaten up twice. The evidence suggested that this was the result of a belief among Hells Angels members that he had had an affair with the mistress of Michael McCrea, the then president of the Halifax chapter," the judge said in his decision

"As a result, Mr. Simmons and his wife left Halifax and spent several years in New Brunswick, returning at the end of 1998."

When the Hells Angels learned about Simmons' return, a hit was ordered, the judge said.

Kelsie's conviction is the second that has been thrown out in the murder.

In October 2016, the Court of Appeal threw out -- for a second time -- the first-degree murder conviction of an Ottawa man, Steven Gareau, who claimed he had no idea Kelsie was planning to shoot Simmons when they went to the apartment building on Oct. 3, 2000.

Gareau, who is now in his early 60s, was first convicted in 2004, but it was thrown out eight years later because of different legal errors by a different judge. He was retried over seven months in 2013 and 2014.

In February, a Nova Scotia Supreme Court judge stayed the charges against Gareau, saying a third trial would undermine the integrity of the judicial process. Justice Jamie Campbell noted Gareau had served 17 years in prison, endured "two fatally flawed trials" and is confined to a wheelchair and in failing health.