Most RPGs tend to advise the GM to minimize the number of non-player characters in the party. After-all, the story is supposed to be about the PCs, and there are far too many horror (or humor) stories about novice GM showing unfair favor to their "sock-puppets". But for the purposes of "Dead... And Back" it may be in the game master's best interest to have multiple NPCs in the party.
Non-players can help bring the group together. Perhaps they're a shared friend, to allow otherwise separate characters to meet.
Conversely, if the players are somewhat loathe to begin inter-party tension of their own accord, the NPCs can be the discordant party.
Killing players is hard, both because they're crafty, and because it takes them out of the game, the whole reason why everyone is together in the first place. Killing and NPC can illustrate danger, without ruining the night, or provide an in-situ back-up for the player who does see their avatar murdered.