I've got a few creepy ideas to add. How about a strange disease spread by bioterrorists that causes a person afflicted to begin decaying and decomposing, WHILE THEY'RE still alive! It can an interesting take on the zombie genre, as it focuses on their physical and mental states going straight to hell before ultimately dying a horrible death.
Or perhaps a quiet, mild-mannered man who is bullied and abused by co-workers at his job at he seems to just sit there and take it unaffected until one day he goes on a killing spree because he reaches his breaking point and no longer values life. He lives alone in his apartment and has very little contact with anyone, rarely speaking, only biding his time until all hell breaks loose.
