Left 4 Dead

Left 4 Dead is a co-operative multiplayer first person shooter in which you play as one of up to four people (called “Survivors”), luckily immune to a virus that is spreading through the human population with frightening speed. The pandemic’s victims become grotesquely disfigured, wildly violent psychopaths, attacking the uninfected on sight.

You can, if you want, decide to play as an infected psychopath, bent on killing the four survivors.