but a single nameless innocent bystander who catches a Cutting Curse

It seems that he promised himself to stop trying to save everyone even if a minor character dies accidentally. In that case it wouldn't matter if he considered himself directly responsible for the death of Rita Skeeter.

You can't do that.

Indeed. I don't see how he could manage not to compromise his 'every human life is precious' principle in a war. He's hesitating between two possible courses of action -- doing the math or playing Ghandi -- and neither seems like a satisfying choice. He really needs to become omnipotent or at least avoid the necessity of making such a choice.