Actually that is EXACTLY what the "rules of war" say. They only last and are effective so long as all parties uphold them and once one party breeches the rules all other parties are now allowed to breech those rules too.

Ukranian soldier posts video of him in a civilian building using it as an ammo depot/barracks? Congrats its now 100% a valid target.

Ukranian fighters post videos parading around POWs, and otherwise mistreating them? Congrats Russia is now completely free to do just about whatever they damn please with their own POWs.

Maintaining the rules of war is NOT about being honorable, or taking a moral position. Its about agreeing to a certain set of rules that all parties are beholden too. You do NOT follow the GC because you are cool, you follow it because if you don't your opposition can also disregard it and then it just becomes a full scale slaughter.