An Alberta judge admitted on Monday he made a fundamental mistake when he found Travis Vader guilty of two counts of second-degree murder using a section of the Criminal Code that was ruled unconstitutional more than 20 years ago, but he will not declare a mistrial in the case.

Instead, Court of Queen's Bench Justice Denny Thomas changed the two convictions to manslaughter in the deaths of St. Albert seniors Lyle and Marie McCann and set Mr. Vader's sentencing for December.

Mr. Vader showed no reaction to the decision, the latest in a case that has spanned well over six years and is far from over.

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"I think the public should be confident that the system is working…," Crown prosecutor Ashley Finlayson said outside court. "Here we have a situation where a mistake occurred. The mistake, in the Crown's submission, was addressed today."

One of Mr. Vader's lawyers, Nate Whitling, said he believes the only remedy is to declare a mistrial, and that the defence will "absolutely" file an appeal arguing for a new trial or an acquittal.

"I think this is just sort of the latest in a series of unfortunate events in this prosecution," Mr. Whitling said. "It should have been resolved one way or another a long time ago."

Asked by a reporter if he thought the high-profile double homicide case is "cursed," Mr. Whitling said: "That's a pretty colourful expression, but it's accurate in a lot of ways."

Mr. McCann, 78, and his wife, 77, were last seen on July 3, 2010, as they left St. Albert in their RV to meet family in B.C. The motorhome was found burning in a campground west of Edmonton two days later. No bodies have been found.

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Mr. Vader, then 38, was quickly identified by RCMP as a "person of interest," and was arrested on unrelated warrants two weeks later. The RCMP named him a suspect the next month, but he was not charged for nearly two years.

Significant mistakes and upsets have happened in the case from the start – from an early failure by RCMP to identify the burned motorhome as a crime scene, to the murder charges being stayed by the Crown shortly before trial in March, 2014, then recommenced later that year, and the flaw in the guilty verdict in September. Legal experts raised the issue immediately after the verdict, which had been broadcast live.

Mr. Vader has been in and out of custody and faced new charges, and has alleged a range of mistreatment and malicious prosecution by RCMP, the Crown and others.

A strong circumstantial case against Mr. Vader emerged during the 11-week trial, which included his blood, DNA, or fingerprints on items belonging to the McCanns, including inside the SUV they had been towing and on Mr. McCann's ball cap, which had a bullet hole through it.

The key to the SUV was found in a truck Mr. Vader had been using, and there was testimony that Mr. Vader, who at the time was broke, drug addicted and desperate, had money and a vehicle the day the couple disappeared. Personal texts were sent from the McCanns' cell phone to Vader's ex-girlfriend, one of which was signed "t."

The defence argued the DNA evidence could have come from other contact, that the RCMP may have fabricated or planted evidence, and that there was no proof the McCanns are dead.

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In a 130-page decision delivered on Sept. 15, Justice Thomas concluded that Mr. Vader killed the McCanns, even though the exact circumstances are not known. With no evidence of planning and deliberation, he did not convict Mr. Vader of first-degree murder, but then wrongly relied on Section 230 of the Criminal Code to find him guilty of two counts of second-degree murder. Section 230 was declared unconstitutional in 1990, but has never been repealed and remains in the Criminal Code with an annotation.

Justice Thomas publicly admitted the error for the first time in court on Monday. Having received both written and verbal arguments from the Crown and defence about the appropriate remedy, Justice Thomas stood by his finding that Mr. Vader killed the McCanns, but substituted convictions of manslaughter, which require a lesser degree of intent.

He said a written decision will be released soon, and set a sentencing hearing for the week of Dec. 12.

Second-degree murder carries a mandatory life term, but sentences for manslaughter can run from time served to life.

Outside court, Mr. Finlayson said the Crown will ask for something "near the upper end of the range." Mr. Whitling said the defence will likely call evidence related to Mr. Vader's time in custody, as being detained in harsh conditions can be a factor in sentencing.

Lyle and Marie McCann's son, Bret McCann, was not at the proceedings on Monday, but said he had been updated.

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"We are very pleased with the decision today, and are comfortable that justice is being served here," he said. "I hope that any sentence or parole eligibility recognizes his total lack of remorse and his lies."

Mr. McCann said he will be at the sentencing hearing, and hopes to read a victim impact statement.