Robert Balbar claimed at a 2009 trial that he killed Heather Hamill in self defence when she attacked him with a machete, but a jury later convicted him second-degree murder.
The trial heard that Balbar told an undercover police officer that he hit the woman several times with a mallet and then when he found she was still alive, he used a sledgehammer.
Balbar's appeal claimed errors made by the judge in her final submissions to the jury deprived him of a possible acquittal or a conviction on the lesser offence of manslaughter.
The appeal panel agreed, ruling that the judge didn't address the state of mind of the accused as it related to self-defence and that Balbar's limited mental capacity was a significant factor in his intent.
The three-judge panel has unanimously allowed the appeal, thrown out the conviction and ordered a new trial on a charge of second-degree murder.