This is possibly one of the funnest things you can do in garrys mod. Best gamemode ever.
A selected few are chosen to be traitor, with the sole goal of eliminating all the innocent people. The innocent people must extirminate the traitors, but they do not know who the traitors are.
When somebody is killed, no one would know until someone examined the body. So using that, the traitors usually aim to kill people in dark hallways. also, when the body is examined it shows If they are innocent, cause of death, or what gun killed them.
Sometimes a detective is chosen: when he examines bodies it shows who was the last person the traitor saw(backstabs would be one way to avoid being caught like this)
detectives can buy special weapons like DNA scanners and bomb defusers.
Traitors can buy weapons like c4, silenced pistol, or a deadly one use one hit kill knife.
It is like a Fps action version of clue. Very fun.
I stayed up till 5:00am playing this once