Toho's kaiju films, and the properties derived from them, are best known for practical effects and giant monster battles, rather than depraved villains. Even in them, however, there are some villains who manage to cross enough lines to stand out from the crowd as truly, utterly, and irredeemably evil.
Ghidorah The Three Headed Monster introduced audiences to Godzilla's archenemy, the titular three-headed dragon, and living catastrophe, King Ghidorah. Reveling in the destruction that he causes, King Ghidorah flies from planet to planet, exterminating life for his own entertainment. Having already annihilated the civilisation on Venus (Mars in the dub), King Ghidorah arrives on Earth, intent on repeating the process. Beaten by Earth's protectors, Godzilla, Rodan, and Mothra, King Ghidorah did not accept his defeat gracefully, and returned again and again, ravaging the Earth on behalf of the Xiliens in Invasion Of Astro Monster, the Space Hunter Nebula M aliens in Godzilla vs. Gigan, and finally the Kilaaks in Destroy All Monsters, which proved to be the Great King of Terror's last stand. Essentially a life-hating demigod in the form of a dragon, King Ghidorah was undoubtedly the very worst that Godzilla's Showa Era Rogues Gallery had to offer, and was responsible for the loss of millions of lives.
Godzilla vs. Gigan: The titular monster, Ghidorah's DragonGigan, was nearly as vicious as his compatriot, burning most of Tokyo to the ground, laughing all the while. What truly made Gigan stand out, however, was his sadistic love of hurting others. During his and Ghidorah's battle with Godzilla and Anguirus, Gigan repeatedly opted to go for torture rather than finishing an opponent off, dragging the other monsters in close and using his buzzsaw to carve pieces off of them, and showing a distinct preference for targeting those who could not fight back, attacking a severely wounded Godzilla while he lay bleeding on the ground, and later, in Godzilla vs. Megalon, teaming with Megalon to deliver a No-Holds-Barred Beatdown to Jet Jaguar. Prone to abandoning his allies as soon as the going gets tough, and hating a fair fight, Gigan was Toho's reigning king of Kick Them While They Are Down.
Mothra: Clark Nelson is an immoral businessman whose only concern is money. He has his men gun down innocent, peaceful villagers to kidnap the twin fairies they were protecting. He then has the fairies perform show after show as he enslaved them and kept them locked in a cage, even erecting a special glass around them to prevent them from communicating telepathically. He sends his men to kill the protagonists and when Mothra starts rampaging, he doesn't see it as his fault, flees from the country (gunning down a policeman along the way) and finally mocks the twins when it appears Mothra's been killed. His most vile action is the torture of a child who was trying to rescue the fairies.
Godzilla vs. Space Godzilla: Most Heisei kaiju didn't have a choice about whether they were good or bad. Not so the case with the wickedly clever SpaceGodzilla, an Evil Twin if ever there was one. Created when Godzilla's cells fused with a crytal alien, the resultant clone arrived in our solar system with two thoughts in mind—Take Over the World and slaughter his genetic donor in the process. Butchering the inhabitants of a space station that he flew past, SpaceGodzilla arrived on Monster Island where he assaulted, and then kidnapped Godzilla's adoptive son, Junior, out of spite. Making his way to Fukuoka, SpaceGodzilla converted the entire city into a series of giant crystal spires that drained life from the Earth and fed it into him, enabling him to become stronger and stronger. Slain by Godzilla and MOGUERA before he could repeat the process in other cities, SpaceGodzilla was described by Mothra's priestesses as an abomination and was as deliberately malevolent as a Heisei monster could get.
Rebirth of Mothra
Rebirth Of Mothra: Desghidorah is an alien dragon who came to Earth millennia ago with evil intent. Having already killed all the Martians, he aimed for a repeat performance on Earth, only to be halted and imprisoned by Mothra. Freed in the modern era, Desghidorah proceeded to suck all the energy out of the region he was in, slowly draining it, Japan, and the planet as a whole, of the ability to sustain life. Brutalizing the current Mothra's larvae in front of her, Desghidorah eventually killed the moth, before being returned to his prison by her son. Heir to King Ghidorah's mantle as destroyer of worlds, Desghidorah was a reminder that in this franchise, three-heads are always more vile than one.
Rebirth Of Mothra 3: Not to be outdone by his expy, Grand King Ghidorah was, like his Showa counterpart, a spacefaring Animalistic Abominationinimical to all life. He was also a Child Hater of truly spectacular proportions who upon arriving in Japan not only tried to obliterate major metropolitan centers, but kidnapped every child on the island, intending to eat theirsouls. Mothra journeyed to the past and slew a weaker version of the dragon who, during his first visit to Earth in the Cretaceous Period, was responsible for the extinction of the dinosaurs, only to discover that in the present, Grand King Ghidorah was still alive and well. Returning to Earth again, Grand King Ghidorah continued with his plan to leave the planet a lifeless husk, starting with the children.
The alien enemy X is a Large Ham who has a Villainous Breakdown after each of his monsters die, but that doesn't hide just how evil he is. The Controller wanted to take Earth peacefully and was simply a Well-Intentioned Extremist trying to save his people. X offs him the moment he gets the chance and takes over, instantly letting loose his army of monsters and begins the destruction of human civilization. While many alien invaders in Godzilla movies have done just that, it's the sadistic glee present in X as he watches the horror he's unleashed that makes it clear that, unlike his more peaceful predecessor, he's a sadist who enjoys what he's doing! When he captures the Gotengo at the climax of the film, he orders his men to slaughter the crew except the ones he wanted personally. Where his predecessor took prisoners even when he could've simply let the victim die, X has absolutely no regard for life at all. During the Final Battle of the film, he takes his time to enjoy the Curbstomp Battle he's inflicting on his opponent.
X's own (literal) Dragon, Monster X, initially seemed to be just another weapon in the would-be conqueror's arsenal, dutifully battling Godzilla and Mothra on behalf of his demented master. Throughout the fight, however, the creature would display a degree of independence and cruelty unknown in X's other slaves, pinning Godzilla down so that he could be slowly cut to pieces by Gigan's chainsaws, and seeming to enjoy the battle. It was only after X and Gigan's deaths, and the revelation that Monster X had not been mind controlled, however, that the being's true evil shone through. Shapeshifting into its real form of Kaiser Ghidorah, the alien dragon continued to try and kill Godzilla, using Vampiric Draining to slowly suck the life out of Earth's protector. Only too happy to try and destroy the world, and of his own free will, Kaiser Ghidorah proved himself every bit as evil as the Ghidorahs who came before him.
Godzilla Gangsters And Goliaths: Ryoto Takahashi is a Yakuza boss and all around bastard. Murdering Detective Sato's partner, Takahashi has Sato dumped on Monster Island, intending that he should die far from home, while Takahashi frames him for the murder of his partner. When Sato returns home with the Elias and Mothra in tow, Takahashi kidnaps the Elias, resulting in not only Mothra, but Godzilla and most of the population of Monster Island rampaging through Tokyo. Killing the priests of Mothra who come to retrieve the Elias, Takahashi demands that the Prime Minister pay him millions of dollars, or Tokyo will continue to burn. His demands met, Takahashi refuses to end the destruction, and attempts to kill the henchmen who say that he should, insisting that he, and not Godzilla, will go down in history as Japan's greatest monster.
Godzilla The Half Century War: Dr.Deverich is in the running for worst human being in the franchise. Having abandoned the Anti-Megalosaur Force (AMF) when they refused to utilize his designs, Deverich sells his psionic transmitters on the black market, enabling terrorists to summon kaiju to any city they want. He levels Hanoi and much of Vietnam when he tests his first device on Godzilla and Anguirus in the sixties, obliterates the city of Accra, Ghana, when he arranges for every kaiju on Earth to arrive there as part of a demonstration to his customers in the seventies, and after perfecting his design in 1987, uses it to reach out into space and summon SpaceGodzilla to Bombay, annihilating it as well. Worse still, the transmitter (accidentally) calls Gigan and King Ghidorah to Earth as well, resulting in the near extinction of the human race in nineties and early 2000s. With a criminal career spanning decades, and a death toll in the billions (millions of which were deliberate), Deverich outdoes every kaiju in the series when it comes to being a monster.
Godzilla vs. The Space Monster: Scott Ciencin's novel saw King Ghidorah in fine form. Arriving on Earth in the midst of a planet killing asteroid, King Ghidorah aimed to reduce humanity to the Stone Age, on behalf of the Space Hunter Nebula D aliens, who feared humankind might someday catch up with them technologically. Freed early on from The Entity who provided his programming, King Ghidorah continued his rampage nonetheless, burning Monster Island and Infant Island to the ground, attacking cities, and cackling like a loon the entire time. Hated by Godzilla because of the joy he takes in violence, King Ghidorah again earned his title as the King of Terror.