Guy Turcotte took the stand at his first-degree murder trial on Monday, saying he was mocked and intimidated as a child and that he had few friends.
"I was sort of the scapegoat," said Turcotte, who is charged in the stabbing deaths of his two young children in 2009.
The former cardiologist's early testimony also keyed in on what he described as a stormy relationship with the children's mother, Isabelle Gaston.
They split up in 2001 after numerous arguments but later made up.
Turcotte's testimony came shortly after one of his lawyers, Pierre Poupart, told the trial he will argue that his client should be found not criminally responsible in the deaths.
While Turcotte, 43, has pleaded not guilty to the two counts of first-degree murder, he has admitted to causing the deaths of Olivier, 5, and Anne-Sophie, 3.
Poupart told the 11 jurors he will call witnesses as well as experts — "doctors specialized in the examination of what goes on in the brain."
"They will talk about an acute suicidal crisis," he said.
Poupart said Turcotte's mental illness brought about that condition.
The high-profile defence lawyer told the jurors they will have to determine whether Turcotte was in a state of such psychological distress. He said he will try to prove that the accused would never have caused the children's deaths had he not been in that mental state.
While the Crown must prove beyond a reasonable doubt that Turcotte was responsible for the crime, Poupart said the burden of proof required to establish a defence of not criminally responsibile by way of mental problems is not as great.
He also reminded the jurors they will need to reach a verdict based on the evidence and not on their emotions.
"Condemning a person who is not criminally responsible would shake the legal foundations and strike a blow to the integrity of the judicial system," Poupart said.
"It would be horrible to be condemned for acts that are not the acts of a person of sound mind."
He warned jurors there are still things to see that are painful and that nobody, even Turcotte himself, can be insensitive to the facts of the case.
The defence lawyer also asked the jurors to not forget that "the children who were victims of this tragedy were also his children."
The Crown wrapped up its case last week after calling 29 witnesses.