I want to clarify something that others aren't. Yes, things that would be war crimes aren't if not used in war, but this doesn't give a reason. A lot of them are to prevent mistakes in war, not because it's evil. For example, you aren't allowes to use chemicals that are often used on civilians, but it isn't because they're particularly harmful. It's because you don't know what they are until latter, so if you see it you may respond with your own chemical weapons, which may be harmful. This is just one example, but a lot of the time "war crimes" are there to prevent escalation, not because of how harmful they are. (Obviously some are because of the harm though.)