Ok so you're losing me a bit. If the idea is for everyone to survival, with dead characters being replaced by new ones, you have a never-ending group survival RP. If you have a secret antagonist, you have a game with an end goal (uproot/eliminate the antagonist). Having a secret antagonist sounds great, but it's no fun if it's permanently one single person. If they die, does it go back to being a group survival RP? And what is the significance of killing people if they just come back over and over?

Suggestion: The group as a whole initially thinks everyone is good, but slowly realizes there's a bad one. They collaborate to figure out who it likely is. The group can make a majority vote to kill a player they suspect. Mix in a lot of arguing and tough choices and moral decisions, and ultimately someone dies. It doesn't have to be a "once per day everyone votes" thing. If they killed a normal character, that player rejoins as a new person and the antagonist continues to be the antagonist. If they kill the antagonist, the player rejoins as a new person and you choose another player to "go bad" and becomes the new antagonist.

A group survival RP with a game element sounds great, but a game needs rules and a way to win. A goal. Something everyone can be trying to accomplish while RPing. Otherwise, people just play until they run out of things to do with their character and leave the RP.