In Rodman Philbrick's novel Freak the Mighty, Kenny Kane becomes known as Killer Kane when he murders his wife by strangling her to death. But even before that, Kenny had made Annie's life a nightmare.
Gwen gives us a hint about that when she tells Max (Kenny and Annie's son) that she didn't see much of her friend Annie after she married Kenny. She says that Kenny made it “difficult for [Max's] mother to have any friends,” suggesting that Kenny was bent on controlling his wife's life completely.
Kenny is actually a psychopath, a violent man whose anger could be triggered by the slightest opposition or by nothing at all. We don't know what triggered his anger against Annie that night when Max was only four. It might have been a comment Kenny didn't like. It might have been a minor disagreement. That didn't matter. Whatever it was, Kenny wanted Annie dead, and that was enough. Kenny always gets want he wants.
Max has long repressed the memories of what he saw that night, but when he witnesses Kenny doing the same thing to Loretta, those memories come flying back to him. He remembers how he tried to pull his father's hands off his mother's neck and how he hit his father as the light left his mother's eyes. He remembers how his father carried him back to his room and told him he was just dreaming. Max even recalls how he broke the window in his room and yelled for someone to help his mother.