I'm not sold on this being completely premeditated - meaning I don't think leading up to that weekend, that TS had a plan to kill him. I think she was definitely resentful of L and angry at being in what she considered a position of babysitter with AS gone so often. I'm guessing she knew or suspected AS of cheating as well and the resentment grew. She may have taken pleasure in hurting Gannon (as an unconscious or conscious way of getting back at both of his bio parents) and lashed out if he did acted up at all. Then, once he was injured badly, she decided she had to be sure she wouldn't be found out and blamed. Her actions afterwards were the premeditation. My guess is that she worried Gannon would tell his dad what she'd done to him and she figured she had to silence him. All so horrible no matter which way we look at it. Wish she'd confess so we could understand but it never brings him back. Just so horrifying.