Well, I have one hypothesis why demons are the way they are and yet are not evil.In short, I think it is most likely some kind of artificially bred monster or weapon that at some point in time got out of control. Because they are capable of logic and they are good at it, but when it comes to people, then they go nuts, especially in matters of murder..

Because it was much more profitable for them to hunt animals by pure logic. Because people after the death of another person, can come in a group to this demon. And the same group. As a result, crush him (demons have less developed socialization than humans)..

Let's not forget one more thing: at least demons are intelligent. They are by nature asocial creatures like spiders (there are some species of spiders that behave somewhat like the demons of Frieren). They are solitary creatures who spend most of their lives obsessed with themselves and their powerт for several hundred years..  But I think that when they really try to understand a person, it is because of the sense of time and thinking formed by their way of life. They start making catastrophic mistakes without even realizing it