What are the PCs going to do? Balk at helping the Ten Towns that feel they have been driven to institute human sacrifice as a desperation measure? It's not like they're gleefully murdering people to further their evil cult.
Yes. That's exactly what my group did.
Because the way it was written in the adventure.
"Why ... why are you doing this?" they ask.
"Uh ... we don't know. We guess it's helping," the Town Councilor responds.
"You
guess?! You are f'ing
murdering people!"
"Yeah, but it might work. We don't really have any proof."
Party huddles together. "Yeah, this guy is clearly insane or evil. Either way he needs to be disposed from power
immediately!"
KILL THE MAYOR!!!!
I would be amazed if 75% of every group playing this adventure didn't have the same reaction. Because that is what is logical to do in the framework of a heroic fantasy adventure game like D&D.
If you are a writer and you have the bald-faced nerve to try to put in "non-evil human sacrifice" and expect it to be only a minor issue in the adventure, you deserve to have your writer's card taken away. It's like casually adding that a lawful good town kidnaps children or practices slavery. Once you do that, and I don't care what alignment the writer puts in the stat block, you're evil - and you're the enemy of a good aligned party.
Putting in a casual "oh, yeah, and they practice human sacrifice" is basically the same issue as "oh, yeah, and there hasn't been a growing season or sunlight in two years." The writers don't think how this would matter in the reality of the characters as actual people. It's as if they didn't even try to present the setting as a real, vibrant place.
It is a bad module. I'm really coming around to that now. I used to just say "it's flawed," but no - I think I can emphatically say "it's bad."