Well, most of the rules aren't about killing per se, but rather wounding. And most of the rules are in place so that the stakes of fighting never gets high enough that we would all have to stop fighting. For example, the Geneva Protocol that bans chemical and biological weapons pretty much because aside from killing, those who were wounded were an enormous burden, and terrible for morale (aside from any ethical reasons).

In that case, losing the ability to use those weapons was deemed to be outweighed by the consequences of having to deal with them being used on you. Also, since they could be potentially huge and unpredictable force multipliers, it made war all the more predictable, which is good from their perspective.

Long story short, nearly all rules of war exist because they are beneficial to abilities of the parties to conduct war. The rules do no exist to make killing fair. They exist so that parties may conduct war in a way they agree with.