The two men convicted of the 2010 shooting murder of Brandon Musgrave have been granted a new trial based on errors in the judge's jury instruction.

Three years ago, Joshua Warner and Tyrone Chambers were found guilty of the second-degree murder of the 18-year-old and aggravated assault of two others during a student house party on Dundurn Street South.

The Court of Appeal for Ontario quashed the murder convictions based on Justice James A. Ramsay's failure to properly instruct what could be inferred from the post-offence conduct of the accused.

"He erroneously left the impression that Warner's and Chamber's post-offence conduct was relevant to all of the offences with which they were charged and could assist them in establishing the 'respective states of mind of each accused,'" associate Chief Justice of Ontario Alexandra Hoy wrote in the court of appeal decision released Friday.

After the shootings, Chambers, 23 at the time, and Warner, 21, fled the scene together and parted ways at the GO station, the jury heard during the trial.

Chambers hid at his brother's "hang out spot" before fleeing to Halifax. He was arrested there in April 2010.

Warner caught a taxi, went to his grandmother's house and eventually fled to Guyana. He was arrested there and returned to Canada in April 2011.

In the appeal decision, Hoy and two other justices argue that some of the evidence of post-offence conduct was "relevant" to the jury, but Ramsay didn't explain this.

Instead, the post-offence conduct was presented as an "amorphous whole" to the jury.

"Normally, post-offence conduct cannot help to determine the state of mind of an accused, and it could not do so in this case," the appeal decision explained.

Leaving that impression could help establish "mens rea," a Latin legal term referring to criminal intent.

Janet Musgrave, Brandon's mother, said word of the new trial is devastating for the family.

"It's always about the criminals; it's never about the victim, the family's themselves. You have to explain things to your grandchildren — we are just devastated."

Janet says she understands the function of the Appeal Court is to protect the innocent.

"That's what it's there for, not when two (people) clearly know what they did, and there's so much evidence to back it up."

In 2013, both were sentenced to life in prison, while Warner was given no chance of parole for 15 years and Chambers was given no chance of parole for 17.

"He took a bullet for making peace," Janet said in 2013. "(Chambers and Warner) got everything they deserved."

Musgrave was killed at a house party of mostly Columbia International College students on Dundurn Street on March 12, 2010.

Witnesses said Chambers did not like the Nigerian music that was being played and began a confrontation with Wesley Abi to change it.

He soon drew his handgun, which was wrapped in a sock. Chambers' friend, Warner, also drew his handgun. Both fired.

Musgrave was shot in the head. Kauner Chinanmbu, then 18, was shot in the wrist and chest. Ted Tsibu-Darkou, 20 at the time, was shot in the shoulder.

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During the trial, Warner's lawyer admitted that Warner had shot Tsibu-Darkou.

The court of appeal also struck down both aggravated assault convictions against Chambers for the attacks on Tsibu-Darkou and Chinanmbu, but only the assault against Chinanmbu was struck down for Warner.

Janet said the news of trial is saddening, but the family will "be there every step of the way."