“…makes it exceptionally clear” yeah… No. The anime and manga time and time show demons valuing other things than “power” all the time.
The mere fact that they have free will makes it completely unacceptable to do a genocide. A genocide, as Frieren wants it, is indiscriminate, meaning that if there are good demons, they too will be executed.
I think that either the writer is stupid, writing clear contradictions into the manga, or he’s smart and think that demons are more complex.
If their only purpose is to kill humans why do they have free will? Why do they have emotions and feel fear and sadness?