It's actually based on a couple different things: an experience the director had in his childhood. The Manson murder's and another murder that took place in a camp ground in the 80's. It's creepy bacause it's plausable and there have been similar sensless evil acts. It's just plain creeeeeepy.
It's not like some way over the top Texas Chainsaw movie. It's more like a group of psychopaths which makes it even more scary, in my view, because there are "people" like that.