Yes, it is a war. Almost all demons they meet (and kill) are soldiers. But Frieren’s philosophy is one of total annihilation. She wants to kill all demons, no matter if they’ve killed or not.

The problem is that the author seems to want to make this philosophy the morally just one in universe while at the same time giving demons the ability to chose if they want to kill or not. I’m more irritated at the author for not thinking things through.