Murder is a Garry's Mod gamemode where a single player is chosen at random to become the Murderer each round. The Murderer spawns with a knife, and needs to use it to kill all the other players without being discovered. The rest of the players need to gather clues hidden around the map to gain a powerful revolver that can be used against the Murderer. But choose your suspects wisely, because killing an innocent bystander will penalise you...
Having Counter-Strike: Source mounted is highly recommended to ensure most textures and models for custom maps are present.
If you do not own the game, you can download and install the game content as an addon from here, instead.