There is no definite good / bad person in a relationship because none of us has the right to judge, when we are not in the relationship and could never see the whole story.

But I think cheating and lying is still wrong. The fact that the cheater might be abused couldn't justify lying and cheating. I have friends who cheated on their partners, I never judged them and saying they're bad people. But if they asked me did they do it wrong? I would still say most of the time there's a better way to handle the issue, although I understand their difficulties.