Tommy owes Lizzie, he put her up to prosituting herself at the end of season 2 when she was finished with it. He said he would make sure he was there before anything happened, he wasn’t there.
He also continues to use her for sex when he know she has stronger feelings for him than he does for her.
The Shelby family tends to go around bullying and hurting other people at a moments notice, I get it’s the “game” they are in but to be honest my sympathy for them is miniscule. This situation is if their own making.