Death in modern warfare is usually fairly stochastic, you know that when a unit goes into battle that a certain amount will fall to bullets and shrapnel, but those giving the orders usually cannot identify the exact men who will die as a result of their orders. That wasn't so much the case in earlier eras. There used to be something that in English was called a 'forlorn hope': a body of soldiers chosen in advance to be the ones that will be given a task that was almost guaranteed to kill them. The first ones over the wall when assaulting a fortified position, the ones on the front rank of an insufficiently fortified position being assaulted, those holding an outwork, things like that. Many military units would go so far as to have a separate unit of condemned criminals to perform these tasks. What is this, if not human sacrifice?
There being ways to translate deaths into military advantage is inherent to this era of warfare. Incarnate Elementals introduces a way to make it so that captured enemies can be the ones to suffer death for your military advantage. That doesn't mean that everyone is perfectly okay with it, but it does mean it's something most military people in the setting can see the logic behind, rather than being seen as unacceptably monstrous.