I have a story idea about a group of friends go on a road trip and discover a hidden town with a dark secret.

Question

The friends will be driving to get to one of the friends' vacation house. While on the way, they stop in a small, dreary looking town because of a flat tire. Everyone there seems.......distant. No one interacts with the group of friends, and the cashier at the gas stations seems scared of them. They soon figure out that the town did something dark and bad together as a community and don't like/are scared of outsiders because they are afraid they will find out their secret.

What did the community of this dreary town do that was really bad? How do the group of friends find out? How do the group of friends deal with their newfound information? No death of ANYONE can be involved or fraud.