I already made something quite a bit like this, which I'm actually hosting right now (Port's Borg TDM).
I'll just explain the rules of the game-mode here so I won't have to explain it over and over on the server:
There are two "teams", the Blockheads and the Borg.
The Blockheads can use weapons, move at normal speed and can only damage themself and the Borg.
The Borgs cannot use weapons, move at a faster speed and can turn Blockheads into Borgs by right-clicking them.
The Blockheads' goal is to kill all Borgs, and the Borgs' goal is to turn everyone into Borgs.
There are no visual diffirences between a Blockhead and a Borg, but Borgs can be spotted by that they are not holding a weapon and move rather fast.
Blockheads should use a searching behaviour, once in a while checking behind their back.
Borgs should use a stealthy behaviour, often taking alternative routes.
