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 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.