Some say this isn't a horror film technically, but I believe with the amount of violence and gore, this can be a horror film.  I speak of "The Belko Experiment", it has all that and more.  The premise is everyone working at Belko has been there many years, and one day it just "happens" to go total lockdown.  Then comes the voice over the speakers, informing them that only one person shall live.  You can use anything at your disposal to kill everyone in the building, only one survivor.  As bodies begin to pile up small groups form, but only one survivor is allowed so.......  Greg Mclean of "Wolf Creek" fame directed this onslaught, which I found very entertaining to say the least.  And not to spoil the end, but it sums up the whole idea, just awesome all around film.  Must see if you ask me.