It's more sci-fi than horror, but the concept of every day citizens being turned into mindless soldiers has been featured in a lot of horror flicks. Well, Square-Enix is going to be announcing a game based on this concept at E3. They've "leaked" a trailer here:

http://www.youtube.com/watch?v=2Ny3CXSQraQ

Personally, it looks like it can be cool. There's zero information out there about it, no name, no plot, just what you see and here on the trailer. I think it would be cool if you could take people over, a la Fallen, and jump from person to person when your current body is about to die. Would be really cool.