Sensei/the Owl Orphenoch is this in spades. It was revealed that he was the one who rigged Kaidou's bike and was the one who ran over his hand, permanently crushing his guitar dreams for good; in said flashback it was shown he did this with a tiny little smile and thought nothing of it. THEN he was planning to do the same to another student only because he wanted to be best at an instrument so he was willing to injure or even kill people in order to do it. This really managed to piss offKiba, the Horse Orphenoch.
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.
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 doing a Heel-Face Turn. Why? Because he has nothing better to do and wants to go to Heaven. AND SUCCEEDS. Good God, Toei, of all the stupidasspulls 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 rather nasty. He strung along people trying to make it in show business, only to kill them and take a prized possession of theirs as a memento. Even Wataru, who had generally tried to find the good in everyone, even Fangires, found his actions unforgivable.
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.
James/Kamen Rider Strike may count as this as well, one factor of this includes the fact he's a complete sociopath and would probably give his counterpart Takeshi Asakura a run for his money and Asakura is in every sense of the word a psychopath.
Jun Kazu, the Utopia Dopant. Now you may ask yourself, "What's so bad about Utopia?" First of all, he has every power imaginable by turning earth into Utopia, which he manages by stealing a person's necessary emotions, like hope and happiness. He then proves how far he's willing to go by setting Accel on fire to the point of causing him to detransform and still be on fire! He then proceeds to steal the faces of everyone in the supporting and main cast except for Wakana, Shoutaro, Saeko, Phillip, and Terui. He then kills the woman he loves. And it takes a nuke and Phillip'sdeath to kill him!
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 blackmailed the woman he loved by turning her love into a jewel, and then tried to kill her when he had no more use for her. Liar may have been made worse by his Gaia Memory but Jewel destroyed the windmills a group of children put up just for fun before becoming a Dopant and was a Complete Monster before getting the Jewel Memory.
Motoko Saeki, Wakana's rival, proves that you don't need a Gaia Memory to be a monster in Futo. She manipulates Wakana's Stalker with a Crush manager into committing massive property destruction and presumably significant 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!He would blast the area with an electric pulse, killing anyone that has not lived up to his standards for the project, referring to it as "cleaning out the test pool". When Daido ruins that plan, he reveals that was merciful compared to what happens if they try to escape. The eyes he implanted on them to watch them kill them when they leave his compound. And as Katsumi is in shock over the slaughter of the entire village he'd tried to save, Dr. Prospect is laughing his head off at the fact he'd just slaughtered a village of innocent people! His actions are so horrific that it drives Katsumi insane! What this monster put Daido Katsumi through was so terrible and so horrific that it actually serves as a valid Freudian Excuse for Katsumi and also means Prospect was responsible for everything that happened in the movie! There's a special place in Hell for this sicko, and Katsumi was more than happy to send him there.
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 used Mezool, the Team Mom of the Greeed, and Gamel, a Man Child who really doesn't know better, as lab mice to see what would happen if a Greeed absorbs Core Medals that aren't their own and wasn't the least bit disturbed when this resulted in their mutual demise. In fact, he took advantage of it to assimilate their Cores and get even stronger. After that, he begins raising Lost Ankh as his own personal Tyke Bomb and actively helps Dr.Maki try to destroy the world. And he did all this with a sick smile on his face (well, in human form, anyway). He's so evil that even his own kind doesn't care about his demise.
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 his dead sister is brought up. However, it turns out what he came to believe were false memories and he actually burned his sister to death in her sleep just because she was getting married and would have abandoned him. 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 absorbing five Purple Core Medals to begin transforming into a Greeed himself. His Yummy so far have been Omnicidal Maniacs and a monster who tears people's dreams apart, leaving them in a coma as a result. What's his reaction to this? He finds it "interesting" and justifies it as For Science! He's just been getting worse and worse since then. Kazari finds out too late that Maki was Eviler than Thou when Maki impales him and rips all his cores out of him, leaving him to die a slow painful (though admittingly well deserved) death. He also gives two of his own Cores to Eiji, to accelerate Eiji's greeedification. Finally, he tosses the remaining Core Medals from the other Greeed into Uva to forcibly set his ultimate plan into motion—much to Uva's horror.
Another Complete Monster is the original OOO. An evil king described as "desire incarnate", his goal was to conquer the world and become an immortal god. To achieve this goal, he had five fully sentient organisms created for the sole purpose of killing them off to reach his goal. He then stabs Ankh, the only Greeed who willingly works for him, in the back for his Core Medals. Worth noting is that he's so power obsessed that he's called Selfish Evil by a living incarnation of human greed! His own greed ultimately spells his end when he can't handle the power he finally absorbed and is killed by it.
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 blew up the Rabbit Hutch locker and rubbed it in that Kengo would be left there to die all so he could see Yuki Jojima, the girl he was currently crushing on, cry. 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 drive itself off a bridge killing all of them 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.
Natsuji 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, admitting after being found out that he was intending to do it to someone anyway For the Evulz and she just happened to piss him off. 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 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.
Tatsumori 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.
The "School War" stunt made him more than a contender.
Rem Kannagi, the Big Bad of Movie War Megamax. His goal is to use the power of the Astro Switches and the Core Medals from the future to become an all powerful being known as the Super Galaxy King and will do absolutely anything to accomplish this, and taking a sick glee in rubbing his advantage in the faces of his enemies whenever he gets the chance. He essentially kills Gentaro's girlfriend, Nadeshiko for the SOLU she's composed of, then rubs that fact in Gentaro's face. He also had no problem killing anyone that gets in his way without an ounce of remorse, or creating clones of all the Greeed and Sonozaki Dopants for his own evil purposes. He gets his in the end though, after being Double Rider Kicked by Fourze Double Rocket States and OOO Super TaToBa Combo, sending him into the path of his own shuttle that runs him over.
And then Leo Zodiarts tops that by beating up innocent patients at a hospital—one of them being Haru—just to lure Ran out and have her transform into Pisces. Then he tops that with the destruction of the Rabbit Hutch, attacking anyone—including Ohsugi-sensei who actually tried to fight back—and leaves all the members in the Rabbit Hutch battered and bruised.
And before Makise, there was Scorpio Zodiarts. Its identity was the sweet Sonoda-sensei who becomes a complete bitch when she decides to give Miura another Switch—and Miura is going through symptoms of Zodiact Switch withdrawal—and doesn't give two shits when she decides to give Gentaro, JK and Shun scorpion poison that will effectively kill them all. As the only female Complete Monster in this series, she did a lot for being the Starter Villain.
Phoenix. While all the other Phantoms (including Medusa) are definitely no saints, Phoenix has been shown to be a complete psycho who will go to horrific lengths in order to make gates create more phantoms. This is shown quite well in episodes 8 and 9 when he beat the holy shit out of a lady just to have a child give into despair; he takes it Up to Eleven in 9 by going the same route as Leo Zodiarts (breaking into the hospital and randomly beating up innocent people, including patients who have no connection to the Victim of the Week at all). Even Medusa is frightened of his tendencies, because he has apparently gone so far as to kill a Gate before the corresponding Phantom can manifest (this is why he is kept to an observatory role at first, rather than being allowed to directly intervene).
Sora/Gremlin may very well count on the fact that as a human, he was a Serial Killer who was responsible for multiple disappearances before he was kidnapped to be made to give into despair. Now normally, one a Human gives into despair and becomes a Phantom they lose almost all aspects of their former personality (take for example Yugo and Misa before they became Phoenix and Medusa) however it seems Gremlin still has Sora's desire to murder women with long black hair and white dresses; when in Episodes 34-35 it actually seemed like he had noble intentions of protecting the Gate however it was revealed he intended to indulge in his host's former taste, attempting to kill her for the sheer fun of it he also makes no qualms about the fact that the women he murdered he did it for fun or petty revenge.