I think I'd randomly assign everyone's character a role, stick them in an old house owned by the family of my character, and let people interact. If someone kills someone, they'd have to do it via PM. Perhaps have interactions that aren't large-grouped based in rooms occur via pm. If someone was killed, then their character will have to stay in that room, dead, but will post they're staying that room, while whoever else was in the room would say they left.
I'd say if the to-be victim would roll a d20 die against the killer, and if the killer wins, they die. However, the killer can only kill once per every two page of posts, I think would work.