There's a part of the story where demons are negotiating peace between a town and attacking demon forces. As soon as Frieren hears of this, she says the town is doomed.
Moral of the story is if your problems are caused by a group, don't expect a member of that same group to help you solve that problem.
Far longer than there have been "humans" there has been slavery. Our pre-human ancestors were smart enough to know that forcing someone else to do hard work for you was a good idea. & etc.
Note: we humans are homo sapiens and there were lots of other "proto-humans" before we arrived.