The Babysitter by Andrew Coburn

You can't "spoil" something that's already rotten. That being said, this post contains some specific plot information about the novel discussed. 

For about twenty pages or so I thought I was really going to enjoy Andrew Coburn’s novel The Babysitter. It has the feel of a literary novel, something like Tom Perrotta or Jonathan Franzen or Bret Easton Ellis might write. Then it turns from