Murder is a Multi-Player Gamemode in which a single player is selected from a group in which to be the Murderer and another is selected as a Armed bystander and the remaining players are mear bystanders.

The Murderers role is to Kill all players without being figured out.
The Armed bystanders job is to figure out who the murderer is and kill them trying to avoid killing a innocent bystander.
Bystanders have no real role how ever they can help the Armed Bystander out or they can cower in a corner to save their own life.

If the Armed Bystander is killed another Bystander can pickup the gun and essentially become the new Armed Bystander.

Murder - Steam Workshop

Murder - WithSix