Maybe you are feeling trapped and uncomfortable in this situation living with your abusive ex- you do not want to face it so you have these punishment fantasies- this was a person who probably made you feel awful and now that person is now living with you still- doesn't matter that you are no longer together or how she is now

It is ok to take care of yourself first and do things for yourself

It just doesn't feel healthy and subconsciously it seems to be still affecting you.

Could you move out or find another room mate?