Alberta's top court has ordered a new trial for a man convicted of second-degree murder after a University of Calgary student was stabbed to death at a 2013 house party.

The Court of Appeal overturned Mitchell Harkes's conviction on Wednesday, saying the trial judge made an error in her instructions to the jury regarding the ways in which he could be convicted of murder in the death of Brett Wiese, 20.

Harkes faced seven charges including aggravated assault, attempted murder and assault, all stemming from a house party in the northwest community of Brentwood in January 2013.

He was found guilty in 2015 of all seven charges.

The new trial is for the second-degree murder charge only. The other charges were affirmed by the Appeal Court.

The party and the attack

During the trial, court heard that Harkes, along with his friend and her twin sister, returned to the party to get revenge after the girls were kicked out of the home earlier in the evening.

His friend, who was 17 at the time of her arrest, was found guilty of second-degree murder in May. She cannot be identified under a publication ban imposed by the Appeal Court.

The Appeal Court took no issue with the facts of the case, including Harkes's admission that he stabbed Wiese multiple times in the back, chest, abdomen and face.

Har﻿kes admitted he had taken a knife from his friend's sister prior to storming the house and getting into several fights with several different people at the party.

Court heard Harkes had walked away from an initial scuffle and then became engaged in a second confrontation with at least two people, when he pulled out the knife and stabbed Wiese six times.

"These injuries were not immediately life-threatening," the appeal judges wrote in their decision.

Wiese staggered away from Harkes and was then "stabbed once in the back by the young offender, a blow that caused a catastrophic severing of his aorta," the judges wrote.

"He died as a result of massive blood loss."

Harkes's defence lawyer told the court he acted in self-defence and had no intent to kill.

Error in jury instructions

In her charge to the jury, the trial judge outlined three possible ways in which they could convict Harkes of second-degree murder:

By acting alone. By committing murder as a party with the young offender. By "carrying out a common unlawful purpose with the principal offender."

Including that third route to conviction is what the Appeal Court found to be the judge's critical error.

The appeal judges wrote in their decision that there was no "air of reality" to support a murder conviction on that basis.

They added that the trial judge failed to "explain what findings the jury was required to make" before arriving at such a conviction, which constitutes "a substantial error."

"In our view, to reach the conclusion that the appellant had the necessary subjective knowledge for unlawful common purpose murder would require a string of inferences that no reasonable jury, acting judicially, would draw on the evidence presented at trial," the appeal judges wrote.

Since it is impossible to know "which route the jury took" to convict Harkes, the appeal judges said a new trial is necessary on the second-degree murder charge alone.