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.
I mean technically, there is STILL slavery in the world. Human trafficking ring is basically slavery, too. Do you know who are most likely behind running trafficking ring?