The jury in the case reached the verdict after deliberating since Friday morning.
Tobby Carrier's attack on March 31, 2009, at the family home in Matane, Que., left his older brother, Ismaël, 21, dead from a stab wound to the heart. His parents, Nelson Carrier, 55, and Chantale Michaud, 46, were each stabbed more than a dozen times.
Carrier talked about himself in the third person throughout his testimony and told the jury he was not aware of what his body was doing during the attacks.
Provincial police officers who testified at the trial described the bloody scene at the Carrier home as appearing like something out of a horror film.
Second-degree murder carries an automatic life sentence in prison without the possibility of parole for at least 10 years.
Experts disagree on Carrier's mental state
A psychiatrist who testified at the murder trial told the court that Carrier was having a dissociative episode the night he stabbed his family members.
Dr. Marie-Frédérique Allard said she diagnosed Carrier with long-term depression and borderline personality disorder – a condition often brought on by childhood trauma.
During a police interrogation, Carrier told investigators he had been abused as a child. He also told them he had stayed up all night playing video games the night before attacking his family members.
Carrier also testified that he had been smoking marijuana at a friend's house in the afternoon prior to the attacks.
Allard said this may have contributed to his deteriorating emotional state, but does not believe he was still under the effects of the drug during the stabbings.
She said Carrier's lack of sleep and severe depression built up to a major depressive episode.
Psychiatrist Sylvain Faucher, who testified after Allard, disagreed with her claims that Carrier was undergoing a dissociative episode on the night of the stabbings.
According to him, the descriptions in Carrier's testimony hinted at his capacity to describe facts, reflect and remember conversations that took place on the night of the attacks. He said these elements were not symptoms of Allard's diagnosis."I find Mr. Carrier gave us many details. For me, it's fairly surprising," he said. "He could understand the nature of his actions and their consequences."