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.
Was the town doomed? Was it doomed because the town tried negotiating, or because the demons were going to eat it anyway? Did the effort to negotiate increase hostilities? Did negotiating remove strategic resources that caused a problem? I think we need to know before we can accurately assign your moral to the story.
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.
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.
The negotiations allowed the infiltration of scouts/spys into the town. They could openly reconnoiter for the upcoming invading army. Or just kill everyone themselves, hard to say.
Once they let them in? The town was doomed if they didn't immediately kill them and (possibly) prepare for the onslaught.
I'm not sure if the Demons ever said if they were advance troops or planning to wipe the town out themselves, just the 3 of them.