Complete Monster/Kingdom Hearts


 * The Kingdom Hearts series has Master Xehanort. He's responsible, both indirectly and directly, for all the hardships the main characters suffer and the creation of the three main monsters that are destroying the worlds. The secret reports in Birth by Sleep paint him as a bit of a Well-Intentioned Extremist, unleashing the Heartless and the Unversed to flood the universe with darkness in order to bring about perfect balance between light and darkness. However, that's in the backstory - in Birth by Sleep, he's clearly Jumped Off The Slippery Slope and whatever good intentions he once had are nowhere to be seen.
 * Describing Master Xehanort in such broad terms makes him seem positively tame compared to his actual portrayal. What makes Xehanort truly awful isn't the amount of harm he causes to the world, but the depths to which he sinks in his treatment of Ven and Terra. His predatory behavior destroys the lives of everyone he comes across, and what makes him even worse is that there's nothing anyone caught in his plans can do about it -
 * That's not the end of it.  The kicker?
 * And then 3D shows that he's even more of a monster than we ever imagined, as the game reveals that
 * Let's not forget his
 * Ansem, Seeker of Darkness is possibly the worst of all his incarnations.
 * And 3D makes it even worse.
 * Xehanort's Nobody Xemnas definitely deserves a mention as well. He ruled Organization XIII with an iron fist and couldn't care less about acquiring a heart like a lot of the other members of the Organization: he wanted nothing but power over the heart of all worlds, and thus the entire universe, and would callously step all over anyone who got in his way. When his plans for universal domination get thwarted, he opts instead to plunge all of existence into nothingness. 358/2 Days actually makes him much, much worse: it's revealed that he doesn't care at all about the other members of the Organization, his ultimate plan is to have them all killed off by the time he reassembles Kingdom Hearts so he can take over without anyone to rival him. It's also bad enough in the same game where he sets up best friends Roxas and Xion to . But it isn't until 3D where we see the man at his worst: Xemnas is the perfect example of the typical Nobody: cold, unfeeling, ruthless, and just plain heartless.
 * Vanitas, Master Xehanort's apprentice, gets a special mention, since not only is he a Complete Monster, he's also.
 * He's a murderous Psycho for Hire, but next to Master Xehanort, he's like the Diet Coke of pure evil, even considering the severity of their deeds only.
 * In Kingdom Hearts II. Scar makes his appearance as the Big Bad of the Pridelands. Pete states that Scar was so evil, he became a Heartless on his own. In fact, Scar's such a monster that he retains his original body when he becomes a Heartless, something only Terranort managed to do. And his evil was so great, it lingered and took the form of his ghost, something that Xehanort also did back in Reverse/Rebirth with Riku. And when all the ghosts made of his darkness merge, they became a HUMONGOUS Heartless. That's right, kids; Scar is as evil as Xehanort, the Big Bad of the entire series.
 * As of 3D, we can add Judge Frollo to the list. Even Riku acknowledges how far gone this guy is.