They were working on a project that only a few of them knew what it really was: a cellular regeneration formula that brought dead organisms back to life (think T-virus from Resident Evil). One of them had been infected with it during testing and was killed by another labtech (who didn't remember committing the murder until just before the Halloween event); he came back at the Halloween party and started biting other people and infecting them with the zombie virus.
yes. and the "vials of unknown liquid", which were sold by that same labtech in the market place [labtech X], were a way to be turned back into a human. now, they just sit on shelves in our houses, i guess.
The labtechs are NPCs, scientist who worked for G-Corp. They were many and all alike each others. They blew up with Gambino's Mansion at Halloween and became zombies, all except two of them, one of them, Labtech X, who doesn't show his face.