Prisoners have no weapons. Guards could have tasers or machine guns. Period stuff would be best, but actually feel is what I'm after rather than authenticity. Every game is team deathmatch mode. You can have guards with the "not-damage-each-other" bit set so the only thing you can do is kill prisoners if you're a guard. Every prisoner dies if map is run to completion. Objective for guards is to herd prisoners into gas chamber. Objective for prisoners is to stay alive as long as possible.
Could be extended to allow graffiti-ing, or game could be modded to do more with bodies once players are dead. Could be videoed for a machinima event and destroyed.