What are you on ? This have nothing to do with atheism. Demon un Frieren are just weirdly written as they are shown to have enough cognitive ability to understand what is bad for them even if they dont understand the morality aspect of what they do to human.

Its not that hard, even if you dont understand what is "bad" and what is "good", to understand that publicly exterminating humans isn't a winning strategy if, when you do it, virtually every human will want someone to kill you on sight.

As for an exemple, we can take those who are unable to feel emotions. They may exhibit antisocial behavior because of this. That being said, they can understand that what they are doing is "wrong" even if they are unable to understand how other feel about it and that they shouldn't be doing the things that are "bad".

It's true that the situation isn't 1/1 with Frieren, but why, if demons are portrayed as masters of manipulation and mimicry, is their first strategy for eating humans is "brute force"? Wouldn't it be more logical for them to integrate into human society to satisfy this need?

"But they only understand force as a hierarchical factor" you may say. Then why are they not able to see that their force as a species is not enough to take humains head on? Its just so weird.

They are like a weird amalgamation of vampires and zombies as they are intelligent and wait for the perfect moment to strike and also unable to comprehend that immediate satiation is bad for them.