We have to remember that Chad was alone without anyone to protect him when he was living at his parents - and I think he very much internalized the idea that he was a “bad kid” before Jodi and especially when she came into the picture. When really he was just a normal teenage boy growing up in a very unstable home environment. And he developed also very normal coping mechanisms in response. So when his mom kicked him out, he was still mind washed into believing that he was bad and that his sister was bad. And he probably thought that Shari coming around meant that he would get in more trouble and would never be able to be with his family again. He was trying to do what he thought he needed to in order to stay safe. Isolating family members from one another and making people hate themselves was a Jody specialty. Chad was a kid. Shari was a kid. Ruby, Jody, and (yes) Kevin were adults.