In Haunt #43 - The Star Sickness: I'm a Traitor and Have the Serum, now what?
You were in a very awkward position: you had one "obvious" thing to do, and if you didn't do it, you would clearly be a traitor. For best results, don't make the serum in the first place. Any advantages you get from remaining hidden for a few turns longer are vastly offset by creating the items that the Heroes require to win. (And you are only going to remain hidden for a few turns; any further requires a Sterling-silver tongue and extremely gullible players.)

You had three options:


Stall for time. Instead of making the serum, you harvest extra mandrake, while claiming that you're harvesting enough for two doses "to save time running around." Depending on how hard it is to get extra mandrake, this may be too obvious of a lie.
Run away. First, walk towards the lab to stall for a turn or two, then run as far away as you can.
MURDER. If you can't plausibly pass off your actions as innocuous (and frankly, holding the serum or enough mandrake to make it means it's very hard), then embrace your Traitor nature and try to infect or kill the un-infected players. If you already have the serum, then bring it into combat anyway. (You could drop it in a distant room, but that will make it too easy to recover.) Your optimal targets will be:

Weak against your attack (Usually Might, but it could be Speed or Sanity with the right items)
Far from other players
Already have some mandrake, so their death/infection will slow down the Heroes' victory