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.
the negotiating demons were an advance force looking to get the town's defenses down so their waiting zombie army could waltz in and slaughter everything alive.