Imagine if you will an elf-form of the Doom Slayer, Frieren knows the tricks of the demons that haunt her world. Much of this series follows the lives of Frieren, Fern, and Stark on their adventures that go across lands. Occasionally on their journey, they will encounter a demon (and they are the most accurate portrayal demons, more on that later) that Frieren cannot ignore. She willingly enters battle with them, resulting in their defeat and erasure from Frieren's world. She also likes to get grimoires and what she sees as treasures.
Treating the demons as beings that thrive on the discord of others is the most accurate portrayal of demons. They are entities of evil, most of what they do causes strife and tears the human soul away from performing or doing what is good. They can hide behind what appears to look good but in reality are nasty beings who are damned beyond redemption. In other series of the Japanese, demons are treating more humanely which negates their namesake and sole purpose. This in return makes evil look appealing and not so bad, which should never apply to demons. The demon is an entity which cannot be sympathized with nor shown compassion; to do so, is already to sell your soul to the devil.
As a viewer, I encourage you to take the depiction of demons in Sousou no Frieren as the best portrayal of what a demon actually is. It is not human, but the worst evils you can think of.