Did Oshawa shooter intentionally leave sister alive?

CP24/ Brad, Adelaide, Joe and Sam Traynor in an undated photo

What could have driven a man to shoot and kill his relatives before turning the gun on himself?

It's a question Durham police haven't answered yet, after the deaths of four members of the Traynor family in Oshawa last week. Loretta Traynor, the mother, was injured when her brother opened fire, but she survived.

But her husband, daughter and two of her sons were killed, leaving Loretta and her oldest son as the only survivors.

Why would her brother do this? There have been reports that there was a battle over their father's inheritance which could have led to her brother being cut out of the will and losing out on the family cottage.

Forensic counsellor Sarah Isaacs told Newstalk1010's The Night Side that Loretta's brother may have deliberately left her alive to suffer through the pain of losing her family.

"As soon as I heard about this story, and I heard about the number of victims and who the victims were, and the fact that Loretta, the mother, only (suffered) I believe at the time I heard it was a leg injury, that wasn't by happenstance," Isaacs said. "That was very much a purposeful reaction by the brother.

Isaacs says this appears to be planned, deliberate and targetted.

"He wanted to commit... collateral damage," Isaacs said. "But it's also collateral psychological, long-term damage to his sister. His sister, at the end of the day, I think was the focus of his behaviour, moreso than her family."