Reason 2 Die was a popular ROBLOX game developed by PlaceRebuilder. It's a strategy game where survivors must fend off zombie hordes while accomplishing map goals. As they progressively unlock Weapons, Items, Armors and Ranks, the Monsters must attempt to prevent the survivors from doing so. Reason 2 Die was a free-to-play, although it contained additional in-game purchases that uses R$ (ROBLOX's currency) in order to obtain in-game bonuses such as Boosters, SecretZombies and Cash. Unfortunately, Reason 2 Die was shut down forever on January 1st, 2017.
An Xbox version of the game can be found here. A fan-made remake can be found here.