Demons in Frieren are categorically evil and are people eating monsters that must be killed in the setting. This sounds like something that should feel terrible, after all, how can all beings be fundamentally evil? And yet it doesn't, to me.
The trick Frieren pulls with the demons is incredibly elegant. They are evil because of what they do and how they behave. They are absurdly hierarchical to a fault. Everything is about survival and accumulating power. But that isn't why they are evil.
Demons in Frieren are evil because they use language to deceive rather than understand. The real kicker is, they prey on the best parts of the human heart. The parts that want to understand, forgive, and see the best in other people. Demons will say anything to get into an advantageous position to get more of what they want, regardless of whether or not they even know the definitions or concepts the words point to. All so they can murder people.
The Frieren demons still deal in temptation too, but it's not the temptation to become a sinner, to give up on your ideals. They tempt you to trust and believe, and the reason they are evil is because they punish that trust and belief.