Which reformed villain do you wish would turn evil again?

It's like a natural law — charismatic villains can't stay evil forever. Eventually, they join forces with the hero, and often start to stray onto the path of righteousness once and for all. But which reformed villain would you like to see return to the dark side?

Please include a picture of this newly redeemed monster, so we can all lament his or her fall from wickedness. Thanks!