Complete Monster/Kingdom Hearts

As to be expected from a crossover franchise made by Disney and Square Enix, the Kingdom Hearts series has many villains. Among them are those select few that have the strongest darkness in their hearts...

Kingdom Hearts II

 * In Kingdom Hearts II. Scar makes his appearance as the Big Bad of the Pridelands. As in his movie of origin, Scar murdered his brother, King Mufasa, in order to usurp the monarchy over the Pridelands for himself, and manipulated his young nephew Simba into blaming himself so that he'd run away and abdicate his birthright, which is the position Simba was in when The Heartless (summoned by the darkness in Scar's heart, no doubt) consumed his world in the backstory of the first game. We see the ruination that Scar has brought to the Savannah he reigned over when Sora, Donald, and Goofy visit the Pridelands, and when first meeting Sora, Scar suggests that his starving lioness subjects should eat Sora, even preparing to attack him and take a bite of him himself. Keep in mind that Sora is in the form of a lion cub for this world and Scar doesn't know he's really a human boy, so he's suggesting that he and the lioness herd cannibalize a child lion for food with no shame whatsoever. During Simba and Scar's showdown, 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 Xehanort managed to do. And his evil was so great, it lingered and took the form of his ghost that plagues Simba, 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!

Kingdom Hearts 3D

 * As of Kingdom Hearts 3D, we can add Judge Frollo to the list. While he's been toned down in regards to his lust towards Esmeralda, he's still as bigoted, hateful, manipulative, and genocidal as ever, with his abusive guardianship of Quasimodo and undying hatred for the gypsies still very much intact. In fact, his plan here is to literally obliterate all the gypsies by using the Nightmare Dream Eaters' power, even openly reveling in all of Paris being set ablaze by said power. Even Riku acknowledges how far gone this guy is.

Kingdom Hearts III

 * Lord Cutler Beckett is the Chairman of the East India Trading Company in the world of The Caribbean. Sometime between the events of KHII and KHIII, Beckett had assumed illegal control in Port Royal and enslaved the heart of Davy Jones, forcing him to kill his pet Krakken and making Jones his personal attack dog of the seas, which in turn invites swarms of Heartless that Beckett also exploits in his efforts to claim ownership of the sea and eliminate all undesirables. From the start it's made abundantly clear that Beckett's prime motivation is power and control of all oceanic trade routes, and as piracy is a threat to that, Beckett initiate a purge of anyone even vaguely associated with piracy with all legal proceedings suspended, and uses the Heartless/Nobody fleets as a scare tactic to keep other ships off his waters. Showing immense hypocrisy, Beckett offers Captain Jack Sparrow, a pirate, a deal in hopes of getting him to sell out his pirate brethren, also attempting to manipulate Will Turner to his side with the promise of liberating his father's crew aboard the Flying Dutchmen to ensure not only that the deal goes through but that he gets to claim Jack's life and cheat him out of the promised reward as well. At the level's climax, Beckett has declared all out war against the pirates and attempts to blow up the ship where Sora, Donald, Goofy, Jack, Gibbs, Elizabeth, Barbossa and all the rest are on, intent on leaving none spared, while smugly remarking "Nothing personal. It's just good business." What makes Beckett stand out as particularly vile among villains in this series is that in the amount of time he was present, he showed himself capable of accomplishing acts of great evil without any personal connection to the powers of Darkness at all.

Manga Adaptation

 * Ansem Seeker of Darkness, the Heartless form of Xehanort, is even more malicious in the manga adaptation than in the original games. The true mastermind behind the Heartless' invasion of several worlds and Maleficent's Evil Plan to capture the Seven Princesses of Heart in order to open the Final Keyhole to Darkness, Ansem Mind Rapes Riku so that he can overtake his heart and possess his body at the climax of the first arc. He stabs Maleficent with a Keyblade created from six of the Princesses hearts in order to destroy her once she's no longer of use to him and then deceives Sora into using that same Keyblade to make what would ultimately be a Senseless Sacrifice, turning Sora into a Heartless and unsealing the Final Keyhole. When Riku's heart fight back from within to stop him from attempting to kill Kairi, Ansem uses his power to banish Riku's heart into the Dark Realm. Ansem then attempts to open the Door to Darkness so that he and his Heartless forces can consume the Heart of All Worlds that lies beyond it, which would plunge all of existence into everlasting darkness, expressing nothing but relish at this prospect. Even after he's been vanquished, a specter of Ansem's darkness lives on in Riku's heart and attempts to play the boy's own emotions against him so that he may be strengthened enough to again take Riku's body and regain a physical existence, even using his shadowy familiar to brutally strangle Riku in order to immerse him in total darkness, vicious to the end. Showcasing no redeeming features and played dead seriously against the manga's usual lighthearted and silly tone, Xehanort's Heartless would do whatever it took to gain ultimate knowledge and power, and to sate his undying hunger for darkness.
 * Xemnas, the Nobody form of Xehanort and leader of Organization XIII, is depicted as a cruel sociopath who rules over his organization without care for any of it's members, viewing his followers as weaklings who are only fit to serve his purposes. Looking to exploit the Keyblade's power to claim hearts for making Kingdom Hearts so that he may purge all hearts and then from nothing remake them all into ones enslaved by his power, he allows Sora's Nobody Roxas and Xion, a faulty Sora replica, to grow close intending to then pit them against each other so that he could judge which Keybearer would be more useful to his plans, even tampering with Xion's form, Mind Raping her and sending her to her death. When Axel is sent to look for the runaway Roxas, Xemnas threatens to eliminate him if he doesn't bring Roxas back dead or alive. He later attacks the Radiant Garden with hordes of Heartless, almost getting Goofy critically injured in the process, just so that Sora and King Mickey can slay them all and give him enough hearts with which to complete his Kingdom Hearts. When Ansem the Wise and Mickey try to stop his plans at his castle, Xemnas orders his Nobodies to kill them, and proceeds to overtake the Great Heart from within in an attempt to reduce all of existence to a state of nothingness and destroy all those in his way. Cold-blooded, pitiless, and utterly lacking his game counterpart's Tragic Villain qualities, Xemnas continuously shows himself to be more monstrous than any other Nobody solely because he finds it in his best interest to be so.