I might've mentioned it here before, but the most horrific novel I've ever read is Off Season by Jack Ketchum. I literally couldn't sleep after I finished it. It's not for the faint of heart in the least little bit. And the version I read was edited by the publisher, removing scenes that were more horrifying than what was in the initial printing. An "author's cut" with the restored scenes came out in 1999, but I didn't get a chance to read it.
Gerard