Why when a guy can be very good?

if a guy can be very good to a women but he choses to take advantage wastching the relationship go off kilter and abuses her in ways... I mean why would he abuse her rather than just being a good person? From my experience its because things really AREN'T ok at home so if he is so smart then why doesnt just stop talking to the abusive parents and move on to a new happy life? Yor new partner didnt do anything wrong. I talked to my exes sister and found out she is abusive personality! I felt really bad for him when I found out, but he just trys to hide it and defend his family. Why? Thts stuff you should be leaving way in the past.