is it fair to help your enemies who ruined your life?

It depends in what way. If they are a threat to the society, let them perish. If they were not deliberately trying to mess with you, it depends on you. Some people are more forgiving than others, but scums should never be tolerated. No, there's no God that will punish them. If they do evil things, they should be taken care of right away.

No, but it could be fun.