It sounds like you're making a social deduction game. You should look for inspiration to the rules of other social deduction games.
It sounds like you also need to think through some pretty fundamental questions about your game, like:
- why can't the killer just kill everyone?
- how can people protect themselves from the killer?
- why would anyone other than the killer want to kill in secret?
but it's good to think those through while also thinking about the essential structure of a social deduction game: alternating phases of conversation and gameplay. "Gameplay" refers to those things which actually change the composition of the game being played, such as one character killing another. "Conversation" is, well, everything else. Classic social deduction games like Mafia or The Werewolves of Miller's Hollow (colloquially, "Werewolf", and I'll be using that within this answer but it's no relation to the World of Darkness games about a similar creature) have a freeform conversation phase and a much more structured gameplay phase. Video game adaptations of the same like Among Us or Fortnite's Impostors mode have a continuous gameplay phase and a more structured conversation phase. Since you're planning to operate in a partly asynchronous environment, freeform conversation is probably a better treatment for that so I'll deal with that going forward.
Werewolf: The Basic Structure
Werewolf begins with an introductory murder, carried out by the werewolves in secret. The town (including the werewolves) then discusses who might be responsible. Once that conversation reaches its end (in online asynchronous games, a set number of hours are allocated for discussion) then the gameplay phase starts, consisting of two parts. First, a suspected werewolf is put to death, based on a majority of the town's votes - though votes don't have to be demanded now, they can be passively collected all through the discussion phase. Then there's a powers phase where characters use their powers in secret. In the basic game of Werewolf, this includes both Werewolves voting in secret to kill another townsperson, and the Seer role learning some other player's role in secret. (The Seer also learned some player's role at the beginning of the game.)
Whenever anyone is killed, their role (werewolf, seer, villager) is revealed to everyone. The werewolves win when the villagers no longer outnumber them, which is easier if they can identify the seer. The villagers win by killing all the werewolves.
Or, in other words:
- the killer can only kill one person per gameplay phase
- people can publicly vote to kill someone per gameplay phase; hopefully they get the killer
- only the werewolves kill in secret, but a player role exists which can act in secret to obtain gameplay-relevant information
Werewolf: The Complications
But that's just the basic game. An easy way to introduce more complexity to that game is to give people expanded roles. Mafia has gone very heavily on these as it evolved over rounds and rounds of play on discussion forums, but even Werewolf had more than just those basic roles. The two villager roles capable of killing in Werewolf are notable for how limited they are:
- The hunter can kill one character of their choice after they die - if they are killed in secret, they do not know who killed them when they make this decision.
- The witch begins the game with a curative potion and a poison potion, each with only one use. Every night in secret she is given the chance to use them, after the werewolves have selected a victim. She knows who the victim is but does not know their role, and while the werewolves always know who they decided to kill in secret, nobody knows the difference between a person killed by a werewolf and one killed by poison.
Obviously you should read up on these games and pick up the mechanics you feel help your scenario the best, but here are my recommendations for operating in an asynchronous environment:
- Limit the total number of gameplay-affecting events such as killings. If possible, schedule them over the gameplay period and make this schedule available to all your players in advance.
- If you let normal characters kill, you don't have to make this capacity available to everyone. Everyone should know the available roles and what they can do, but not all roles need to be in the game.
- If you let normal characters kill, heavily consider restricting the total number of times they can do it, the circumstances under which they can do it, or both.
- Also consider creating roles with abilities to obtain definitive game information on a "gameplay-affecting event" scale, though there's nothing to stop anyone from claiming to have obtained definitive information, since powers operate in secret.