In Frieren's Universe Demons are always chaotic evil because they have a different morality system and evolved from predators that feed on humans by mimicry. They are unable to see humans as anything else but food, and when they communicate they always have an ulterior motive. This demon in question tried to pretend to sue for peace with the humans while she accumulated a puppet army of humans and undead. I think is one of the most original aspects of the series how utterly incompatible Demons are from humans and how monstrous they are.