I think that demons aren’t necessarily one-dimensionally evil in Frieren, but are more meant to be incomprehensible and alien. I also think that if they were to add too much complexity to the antagonists in this story, it risks taking away the focus on characters that the author wants to focus on. At the end of the day, we (the reader/viewer) are meant to be on a journey of self-reflection and discovery with Frieren as she learns about humans, her companions (past and present), and what she feels about them. Other things in the story should complement that end goal.

That being said, I understand the criticism. The nature of demons and how they interact with humanity is a recurring theme in this series. Without spoiling anything specific, the later parts of the story dealing with it do offer more complexity in human/demon interaction. I am not sure how far this adaptation will make it through the story, but I think these demons, being our first exposure to them in the story, is meant to be a bit more clear-cut morally.