Goge Vandire

Jackass who took control of the imperium by killing the high lords and taking the imperiums assassins, causing the Age of Apostasy. Murdered so many innocent Imperial citizens in a fit of paranoia that even the Imperium (the Catholic Nazis of the setting) views him the same way non-Nazi people in real life view the actual and universally despised leader of the Nazis. Took a shitload of spes muhreens to take on the faggot, and even then he only died because his female bodyguards finally realized he was an asshole and cut off his head, possibly by order of The Emperor himself. At least he left behind some epic last words of "I don't have time to die; I'm too busy" which probably was adapted into some nice comedy movies for the Imperial citizenry (those that can afford to watch movies or study history in the first place, at least). A good example of how no matter how bad the Imperium gets, it could always be (and has been) worse.