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 demons are consummate liars. They don't even understand the concept of family, but they realized that pitifully crying out 'mother' when about to be slain can cause humans to hesitate, or even be merciful. The peace negotiations were a trojan horse, a chance to fortify their forces, implant saboteurs, assassinate leaders, and undermine defenses from within.