There are no rules in war, or anywhere else. There are only actions and consequences.
There's no rule that says you can't slaughter civilians by the thousands, torture prisoners until they falsely confess to whatever you want them to say, destroy infrastructure leaving cities with no water, power, schools, hospitals, roads, agriculture or communication. There's no rule that says genocide is not allowed.
Yeah, yeah, there's the Geneva Convention. Whatever. That's not rules. That's ink on paper. It's only as real as we decide to make it.
But sometimes the consequences of extraordinary cruelty in war become more tangible. For example, you'll make more enemies. Mind you, if your goal is to profit from war, that makes it a win-win for you.