Dr. Scott Theriault of Halifax says the accused killer was mentally ill at the time Fred Fulton and Verna Decarie were stabbed to death in their Minto, N.B., home.
He says Despres, who suffers from paranoid schizophrenia, thought he was a military assassin at the time of the deaths and therefore would not be able to appreciate the wrongfulness of his actions.
Theriault adds that Despres has not admitted to the killings.
Despres, 25, is on trial for the first-degree murders of Fulton and Decarie, whose bodies were discovered in their home in April 2005.
Both had been repeatedly stabbed and Fulton had been decapitated.
