Based on your request for a movie that has you wondering about the antagonist, I think you might dig Surveillance (2008):
http://www.imdb.com/rg/em_share/rt_i...itle/tt0409345
Inside is a great flick, but if you aren't a fan of gore and violence, I say stay away. It's not a torture flick, if that's what you're concerned about; just very violent and absolutely intense .