Every great Overlord has had his opposite. That one person who embodies all the good to counteract all the evil contained in one man. If any one person can end your reign of terror, it is this nemesis. It is obvious then, how important it is to mark out at an early time, just who your personal nemesis is. Once identified, you do not want to kill them. Nature will simply raise up another to take his place. Unfortunately it is the natural order of things. However, if you can subvert this person ever so slightly? I am not saying expect miracles, but the impossible begins to be possible. Besides, if nothing else, you know who you need to spy on, and who to keep under a constant surveillance.
I know who my nemesis is, and let me just say, I have corrupted them enough that I am already getting away with a lot! Pretty soon and I will have my nemesis actually helping me out.
Never underestimate them though! Always be on the guard. They can always revert back to all that goodness. Do not let them stab you in the back. Survival is always first and foremost! Keep a wary eye!