There are very clear rules for war as set out in the Geneva convention. The British armed forces also have very clearly defined Rules of engagement. I am sure that the armed forces of most countries that have signed up to the Geneva convention also have clearly defined rules of engagement. There will always be examples of people who feel the need to flout these rules as they seek to further their own, often selfish cause. These people, when caught, are prosecuted as war criminals as they have broken the rules (laws).
As I said before this is just a game, but there are some here for whom it seems to be all consuming and will spout whatever random bile they chose to justify their actions.