Nearly everyone regardless of what side they're on, regardless of what fight they're fighting, believes that they are on the side of good. And most people believe their opponents will use every dirty tactic and nasty trick in the book to win, after all they are evil. Too many use that as an excuse to do ill themselves – in the name of "good" or "freedom" or "justice" or whatever banner they're flying. The mentality is that it it's okay if "our side" does less than ethical things, because we are the "good guys" and it is important that we win. However, since everyone believes they are the "good guys" – if you make an exception for the "good guys", you're making an exception for everyone.