Nancy was injecting Daniel with growth hormone and steroids to make him grow bigger. Daniel got severe roid rage, tied up Nancy, and choked her to death with a cord before passing out. Benoit came home, probably with a toy for Daniel and a bunch of flowers for his beloved wife, and saw what had happened. He killed Daniel to spare him a life of misery and regret. And then he killed himself, worrying that people would attribute the crime to him.

Either that or it’s all a work. Nothing else makes sense.