"I feel horrible. I feel horrible to the point … I can't tell you the feeling in my gut," said the former Mountie, who cannot be identified because of a publication ban.
Canada's top court upheld a decision ordering a new trial for Hart, who had confessed to murdering his twin three-year-old daughters to the Mountie, who had pretended to be an underworld boss wanting to know whether he could trust Hart with criminal tasks.
The court also took aim at the so-called Mr. Big scenario, in which police have pretended to be criminals in order to lure suspects into confessing crimes. The technique has also led to the discovery of missing bodies.
According to the Supreme Court of Canada decision, the RCMP may have abused its power, making Hart's confession unreliable. The Crown's case against Hart in his 2007 conviction was based largely on evidence collected through the sting.
In an interview with CBC Radio, the retired officer said he still believes the investigation produced a meaningful confession.
"To believe Mr. Hart is going to get out of jail and walk away with killing his two twin daughters — it's just overwhelming in Canada they'd allow that to happen," he said.
Hart's lawyers say they feel vindicated by the Supreme Court of Decision, having argued that the RCMP manipulated an impoverished, naive man with a Grade 5 education into believing he would become rich if he told a supposed crime lord what he wanted to hear.
Hart led 'through the looking glass'
In the court's decision, Justice Andromache Karakatsanis wrote that the "state conduct in this case was egregious" against Hart.
"The police led Mr. Hart through the looking glass into a parallel universe where, for many months, they employed extensive state resources to prey on his lack of education, intellect, and life experience; his social isolation; and his extreme poverty," she wrote.
Justice Michael Moldaver was also critical.
"The circumstances left the respondent with a stark choice: confess to Mr. Big or be deemed a liar by the man in charge of the organization he so desperately wanted to join," he wrote.
"In my view, these circumstances, considered as a whole, presented the respondent with an overwhelming incentive to confess - either truthfully or falsely. Having determined that the circumstances in which these confessions were made cast serious doubt on the reliability of the respondent’s confessions, the next question is whether these confessions contain any indicators of reliability. In my view, they do not."
Scenario was necessary: retired officer
But the retired Mountie said playing Mr. Big was necessary in the Hart case.
There were no witnesses to what happened at Gander Lake in 2002, where Hart's twin daughters Krista and Karen drowned. Hart had claimed he had driven back to town for help, although police were skeptical of his claims.
The retired officer — who still believes that Hart is guilty of killing his daughters — said he believes Thursday's ruling to lead to appeals in other convictions.
"They'll examine a lot of them and decide if they can be pursued. But it's what they do and it's what they believe in. Let them believe in it and let them try it," he said in an interview with the Central Morning Show.
The retired officer said he thinks police investigators will adapt the technique to abide by the limitations the court has placed on the Mr. Big scenario. However, he said he fears that RCMP managers will be reluctant to use it, and that guilty people will not be convicted.