In Face/Off, the truth was that Castor and his brother Pollux had planted a bomb somewhere in L.A. and only they knew its location and how to disarm it. With Castor seemingly a vegetable, Sean Archer entered a top-secret prison posing as Castor Troy, complete with surgical alterations and Castor's own face in place of his own. As soon as Archer uncovered the bomb's location from Pollux, his mission was complete. But that's when Castor Troy unexpectantly woke up...
Castor would undergo the same procedure Archer did, this time taking Archer's face and assuming his identity. As a top federal agent, "Sean Archer" would be unstoppable and hold significant power, taunting the real Archer and leading raids against his terrorist rivals; at one point, he even slept with Archer's own wife. As a notorious terrorist, "Castor Troy" would be locked away in prison forever. Until he breaks out, of course.
With murder, kidnapping, and many other forms of terrorism under his belt, Castor Troy was already a very clever and extremely aggressive sociopath, but having his face stolen truly put him over the edge. Castor had once tried to assassinate Archer, but accidentally killed Archer's son in the process. Castor couldn't understand why Archer would take things so personally, but he would when Archer dropped Castor's own brother from the top of a building to his death. Castor's ability to withstand pain and improvise in many deadly ways certainly makes him a vile entry on the Mofo scoreboard. But oddly enough, some people will only remember some of his most evil deeds... committed by Sean Archer.