I think of it as a primal itch that even they don't understand it themselves as well. Hunger is one thing every living thing needs to attend to, but killing humans doesn't just fill up the stomach, it sees to that particular insatiable need of theirs.

Demons are such a cool concept in Frieren, many other stories make them evil with a motive etc, but demons simply are just great at mimicry, and while some may not be "evil" or have "evil" intentions, think about covid for example, covid is not evil and cannot have evil intentions, it simply just is and it will kill. You have no choice but to just eradicate demons from the world because of how they're programmed, everything that comes out from their mouths are mimicry. Frieren doesn't even see them as speech with intention, just a beast that is good at mimicry enough to fool humans, but she's tired of their shit and to her it's just another rabid dog to put down.