So, the heroes have finally captured Duke Diabolico, yay! ... so now what? He's repeatedly proven he not only doesn't want to reform, but will turn any such attempts into another dastardly scheme by masquerading as reformed, and the Cardboard Prison is looking especially porous right now. Plus, he's proven smart enough to defy being a Self-Disposing Villain and even if the heroes weren't too principled to kill, he's also avoided giving the heroes (and other villains) reasons to kill him. Examples of Brainwashing for the Greater Good include: