First of all, Dr. Issacs was bitten par his super zombies in Las Vegas,. Second, he managed to escape Las Vegas, but when he got back, he injected himself with the Anti-Virus. Third, Alexander Slater killed him, but not really and Issacs turned into into what too be a Tyrant. Fourth, he killed all of the Umbrella Staffs. And finally, Alice and Tyrant Issacs fight to the end and Issacs was sliced into pieces par the security laser in the laser room from the first resident evil movie.
