A zombie-like survival scenario
Of couse that's good, but I think "zombies" must be players from begin, not only after death.
But better use Vault 13 map or something, take there like 30-40 players, then GM saying to one random player that he is a maniac and must kill all, then to other one and, may be, 3rd guy. Then asking what weapon they want (SG/BG/EW/Melee/Unarmed/Throwing) and of couse let every maniac think that he is
the only one maniac there and everyone else just vault dweller with mausers, rocks, may be something better, but not serious weapon. It'll be scary for both: victims and maniacs, because when maniacs will see someone dying not by them (especially if body melted or bursted or other gore death), they will start to panic too ^_^
Of couse for survivors' interest reward in the end must be dependant on quantity of survivors, so if almost everyone is dead they will get almost no reward, opposite all maniacs will get some. It'll force vault dwellers to not kill everyone on sight, but only suspicious persons ^_^
After killing a maniac, GM will say "Oh noes! but there's another one maniac!", after finding and killing 2nd, "Oh, there's other one too!".
One of maniacs can even get in trust with victims, he can even be thier leader, and then by killing all suspicious victims kill others when he won't get much resistance.