Killing Floor

Killing Floor is a coop survival game from Tripwire Interactive, based on the Unreal Tournament 2004 mod of the same name.

A government research facility was attempting to create mindless super-soldiers, similar to what commonly appears to be zombies. However, control over the specimens were lost and they broke free. Since the police and first line of military were defeated, you had no choice but to clear the zombie infestation.