Complete Monster/Kamen Rider

Kamen Rider Ryuki

 * Although arguments could be made for about half the cast, Takeshi Asakura/Kamen Rider Ouja is the standout example. Throughout the entire show, Asakura practically exists to murder people and look cool doing it. Not only does he rack up the highest body count amongst the Riders, who are pitted in a battle to the death where There Can Be Only One, but he's also responsible for the murder of an innocent woman (who turns out to be Kamen Rider Femme's sister, thus making getting revenge her reason for entering into the Rider Battle), and, to top it all off, he murders his own estranged brother by feeding said brother to his Contract Monster, Venosnaker...After revealing that he was the one who set the fire which burned down their home, killed their parents, and separated them during their childhood in the first place.

Kamen Rider Blade

 * Tennouji unleashed the Undead upon the world, attempting to manipulate the battle by creating the Trials and Titan, and creating/merging with Kerberos in order to become the genetic template for a new world. It takes all four Riders to stop him in his transformed state, and some of the more sinister Undead (and Joker) are ultimately less evil than he is.
 * Another qualifier for this page is the Peacock Undead. He got Tachibana hooked on the Deadly Plants, and once his girlfriend attempts to come to his aid, he coldly kills her and leaves Tachibana an emotional wreck without a hint of remorse. He also brainwashed many people that he was going to use as expendable slaves to create his "Ultimate Rider"! He's just as bad as Tennouji.

Kamen Rider Kiva

 * Rook, who spends all of his free time making up "games" for himself that generally go along the lines of, "Let's see if I can kill X amount of people wearing Y in Z amount of time." He has a lot of free time.
 * He ends up . Why? . Good God, Toei, of all the stupid asspulls to do...
 * To say nothing of the fact that he is responsible for the near-genocide of, at the very least, the Wolfen, as well as the death of Yuri Aso's mother, Akane. His "death" at Yuri's hands (and his total annihilation at the hands of Megumi) were quite karmic indeed.
 * The 1986 King was pretty monstrous himself, albeit on a more personal level--forcing Jiro (the last of the aforementioned Wolfen), Ramon (the last of the Mermen), and Riki (the last of the Franken) to become Empathic Weapons and developing a full-blown hate-on for Otoya, up to the point of threatening his own son's life if his wife sees Otoya again...the same woman whom he told was not worth killing because he had no feelings for her.
 * The King's actions were so horrible that Kivat II, who is meant to be the King's loyal Transformation Trinket, is so horrified by King's threat to murder his own son that he pulls a Heel Face Turn to let Otoya become Dark Kiva to defeat him.
 * The Rhino Fangire was a rather nasty bastard. He strung along people trying to make it in the show business, only to kill them and take a prized possession of them. Even Wataru, who has been known for trying to find the good in Fangires, found his actions unforgivable.

Kamen Rider Decade

 * Apollo Geist in this incarnation drains the life out of people to extend his own and has been working to converge the various worlds to facilitate Dai-Shocker's invasion of them, eventually becoming powerful enough to wipe entire worlds from existence--which he does with glee.

Kamen Rider Double

 * The Sonozaki Family seemed to be this—headed by an apparent Evilutionary Biologist Corrupt Corporate Executive who gives powers to not-very-nice people and children in what sparks his son-in-law's Heel-Face Turn and subsequent demise. The patriarch's own powers can be described as weaponized horror. While they arguably fall short due to being Drunk on the Dark Side (in addition to having humanity's survival at heart), it's not for lack of trying; anyway, the fact that they gave a few of the people below the Gaia Memories that enabled their rampages without regard to the well-being of others arguably justifies their classification as monsters by association.
 * Liar and Jewel are both this. Liar's motivation was to collect the tears of his victims by destroying their dreams. Jewel turned people into jewels for no reason except for fun and  Liar may have been made worse by his Gaia Memory but Jewel   was a Complete Monster before getting the Jewel Memory.
 * To prove that you don't need a Gaia Memory to be a monster, Motoko Saeki, Wakana's rival, manipulates Wakana's Stalker with a Crush manager into committing massive property destruction and very likely not-altogether-insignificant loss of life (and in one case, crushing Wakana herself--although she gets better thanks to the Claydoll's power) simply to force her off the air. It falls to the resident Anti-Villain to teach her the error of her ways with extreme prejudice.
 * Doctor Shinkuro Isaka, the Weather Dopant. He killed Terui's family, modifies Gaia Memories to kill their users so that he can take their power for himself, and is able to use Gaia Memories via direct insertion without any ill effects (typically, repeated use of Gaia Memories via direct insertion warps the user's mind, which shows that Isaka didn't have far to fall). He then tries to get Saeko to kill her own father and take control of the Museum, but winds up getting Hoist by His Own Petard when all of the Gaia Memories he absorbed essentially destroy him from within upon Memory Break.
 * All of the above are blown out of the water with a nuke by Dr. Prospect from W Returns: Eternal. This man took people from all over the world (their words implying they were kidnaped) to make them into, in his own words, "lab rats" for his Quark Project, giving them psychic powers. The fact that, when he arrives on the scene, every one these people with awesome Psychic Powers is quaking in fear of him shows how horrifically he must have treated them. But that is nothing compared to his Heaven's Fall plan. This is a plan so horrible that the previously mentioned Utopia Dopant considered it a brutal act! When Daido ruins that plan, he reveals that was merciful compared to what happens if they try to escape.
 * All of the above are blown out of the water with a nuke by Dr. Prospect from W Returns: Eternal. This man took people from all over the world (their words implying they were kidnaped) to make them into, in his own words, "lab rats" for his Quark Project, giving them psychic powers. The fact that, when he arrives on the scene, every one these people with awesome Psychic Powers is quaking in fear of him shows how horrifically he must have treated them. But that is nothing compared to his Heaven's Fall plan. This is a plan so horrible that the previously mentioned Utopia Dopant considered it a brutal act! When Daido ruins that plan, he reveals that was merciful compared to what happens if they try to escape.

Kamen Rider OOO

 * Kazari, the Feline Greeed, is a fine example of how a Magnificent Bastard can also be a Complete Monster. Of the five original Greeed, he has by far the most horrifying way of making Yummy, implanting them in their host and controlling their body like parasites; in some cases, the host is conscious the entire time. In the end, they're consumed by it and trapped inside, fully aware of what's happening. If that weren't bad enough, Kazari's actual actions are much worse. He And he did all this with a sick smile on his face (well, in human form, anyway).
 * Dr. Maki himself also qualifies, but is an interesting version of this as he didn't start out a monster but grew into one through character growth. He does some truly horrible things early on, like helping a Mad Bomber who is hosting one of Mezool's Yummy carry out his attacks to see the Yummy grow, not caring who dies in the process. When the swarm of Yummy hatches, he responds by locking his workers inside with them to see what would happen. Predictably, they slaughter the workers, which Maki doesn't care about. He then one-ups himself by joining forces with the aforementioned Kazari. Later, it seems like he might find redemption when After he realizes this, he loses all the humanity he had left and willingly sides with Kazari to attempt to destroy the world. This is where he flies past the Moral Event Horizon by  He's just been getting worse and worse since then.
 * Another Complete Monster is the

Kamen Rider Fourze

 * Minor character Hiroki Makise/Pyxis Zodiarts, who was a Monster of the Week without a doubt qualifies as a Complete Monster. Makise was a psychopathic stalker who kept a journal of the girls he was stalking and sent loads of messages, love letters and gifts to these girls, but what really pushes him past the Moral Event Horizon was when he The next MEH was his ultimate plan for a bus full of girls who found him (justifiably) creepy, his plan was to use his Zodiarts power to manipulate the bus to  the sheer joy he had causing these problems was staggering, seeing as how other Zodiarts have at least some sort of sympathetic reason for becoming Monsters of the Week Makise's excuse is cold blooded and sadistic revenge.
 * Kijima/Pegasus Zodiarts/Cancer Zodiarts could also qualify, Kijima is a Smug Snake who became a Zodiarts because the new teacher confiscated his fan and he decides to frame her by imitating martial arts moves. He also likes to rip out peoples souls by putting them into a "Comedy in Hell" routine; if you make him laugh, you get a Zodiarts Switch and if not, he steals your soul! The worst part of it was that he was doing all of this with a sick cheery smile on his little Troll face, and he gleefully adressed it as if it were everyday business, these were his fellow classmates mind you and he did this to innocent people willfully like The Sociopath he's known for being. He got what was coming to him in Episode 28 by Kamen Rider Meteor and Libra Zodiarts.
 * Yamada/Aries Zodiarts is a contender. He was transferred around the same time Ryusei came to Amanogawa and in that amount of time, he's the undisputed king of Subaruboshi high. He forces the whole school (teachers included) in his little play like world where everyone is graded on their performances. If they did bad jobs, he would them into a permanent sleep. He even makes a Deal with the Devil with Meteor, reviving Jiro if Meteor killed Fourze and he follows up on his deal. Then he decides to do a play called "School War" which has the Kamen Rider Club kill students before deciding to reverse the roles and have the Kamen Rider Club being tied up to poles and killed. Thankfully, Gentaro gets revived and kicks his ass so badly that he gets sent into a coma.