HALIFAX -- The Nova Scotia Court of Appeal has thrown out -- for a second time -- the first-degree murder conviction of an Ottawa man who claimed he had no idea a drug-world associate was planning to shoot someone in the head.

It was a Hells Angels-ordered killing: The dead man, Sean Simmons, had been targeted in October 2000 because he'd had an affair with a girlfriend of a member of the outlaw gang.

Steven Gareau was one of four men convicted of first-degree murder and conspiracy to commit murder.

The appeal court ruled Wednesday, though, that the judge presiding over Gareau's trial mistakenly allowed the jury to hear about a second, earlier shooting, and mishandled wiretap evidence suggesting Gareau wasn't aware of a murder plot.

"In my respectful view, the judge erred in law," said Justice Joel Fichaud.

Gareau has been ordered to face a third trial -- although the court ruling pointedly noted a retrial is "at the Crown's discretion."

Gareau was first convicted in 2004, but it was thrown out eight years later because of different legal mistakes by a different judge.

He was retried over seven months in 2013 and 2014. The Crown's key witness said Gareau was not the shooter, but was aware that the gunman, Dean Kelsie, planned to kill Simmons over Simmons' 1994 affair with the girlfriend of Hells Angel Mike MacRae.

The appeal court ruling said Simmons had fled Nova Scotia for New Brunswick after being severely beaten twice, but "four years later he made the mistake of returning to Nova Scotia."

In 2000, Gareau and two friends, Paul Derry and Tina Potts, themselves moved to Nova Scotia from Ontario, making a living through bank card fraud and selling drugs. Simmons had hoped to sell quaaludes for Derry.

Gareau got his drugs from Derry and another man, Wayne James, who were supplied by Neil Smith of the Hells Angels. When Smith heard about Simmons being back in Halifax, he told James and Derry he'd knock $25,000 off the $85,000 cocaine debt they owed him if they killed him.

"Smith was perturbed by Simmons' earlier relationship with a club-member's girlfriend. So Smith ordered James to have Simmons 'whacked'," the appeal court recounts in its decision. "James then turned to Derry, who ... said 'You heard him.' In return, Smith would waive $25,000 of the Derry/James debt for fronted cocaine."

Gareau went along as Kelsie, an associate of the crew, met Simmons in the lobby of a Dartmouth apartment building. Kelsie pulled out a gun and shot Simmons in the head, which Gareau's lawyers said came as a shock to their client.

The jury convicted Gareau again in February 2014.

On Wednesday, the appeal court said the trial judge made two legal errors, including allowing Derry to testify about an earlier shooting involving Gareau and the rest of the crew: "The message to the jury was: Gareau had the disposition to knowingly participate in a shooting with these same co-conspirators."

The appeal court also said the trial judge was also wrong to disallow cross-examination of Derry on a wiretaped conversation he had with Gareau, in which Derry seemingly agreed that Gareau "wasn't aware" of the murder plot.

"This reasonably might have altered the outcome" of Gareau's trial, the appeal court said.