The Law of Armed Conflict exists because you can't stop war from happening, especially if you aren't willing to fight a war yourself to stop it.
It's also worth pointing out that war/conflict are not the same thing, the definition of who is protected during a conflict is much broader than members of the armed forces during a formal war.
Sure, but "war crime" is a phrase with a particular history and usage that means something more specific. Even opposing sides in war often agree on some boundaries.
