Stephen King has written some pretty nasty villains in his day, but these particularly stand out. With rare exception, the actual monsters of his works tend to pale in comparison to the more human villains.
Randall Flagg is a demonic, quasi-immortal being who frequently appears throughout the works of Stephen King as an embodiment of evil. Living through different lives in different worlds, the one constant about Flagg is that he is always working to sow the seeds of chaos and despair wherever he goes. Flagg was part of the Vietnam War, Klu Klux Klan lynching, police murders, and race riots, and each time he was part of a violent experience it would ultimately serve to empower the evil within him.
In The Stand, Flagg appears as a Dark Messiah in a post-apocalyptic, plague-ravaged United States. Here he builds a new civilization in Las Vegas, calling to him those with penchants for destruction, power and fascism. Flagg has people publicly crucified for opposing or failing him, murders his pregnant girlfriend for enraging him, and plans on destroying the peaceful "Free Zone" settlement just so his civilization can be the dominant one.
In The Eyes of the Dragon, Flagg is an Evil Sorcerer in the medieval country of Delain. Here, Flagg has Queen Sasha murdered, poisons King Roland, frames Prince Peter for the crime, and then plunges Delain into a new Dark Age by manipulating the remaining heir, Thomas.
In The Dark Tower series, it is revealed that Flagg has lived for so long and accumulated so much power that he has become the emissary for the Crimson King. Flagg earns Roland Deschain's undying hatred for beating and sleeping with Rolandís mother, and for aiding the revolutionary, John Farson, in causing the destruction of the city of Gilead. Flagg also forces Roland to let his preteen companion, Jake Chambers, die, and drives a girl insane by telling her the trigger phrase which causes a formerly dead man to recount the afterlife to her. Corruptive, treacherous and sadistic, Flagg's ultimate goal was to betray his master and climb to the top of the Dark Tower in order to become God of all.
William "Wild Bill" Wharton, from The Green Mile, is a Psychopathic Manchild awaiting execution at the Cold Mountain Penitentiary for murdering three people in an armed robbery, one of whom was a pregnant woman. When he first appears in the prison he manages to convince the guards he's in a drugged stupor, only to attempt to strangle Dean Stanton to death when his guard was down. Failing in that, Wild Bill contents himself with causing as much mischief as he can before his eventual execution. Eventually it comes to light that Wild Bill's worst known crime was the rape and murder of the two little girls that John Coffey is accused of killing. In order to stop the girls from calling for help, he told them that if one of them screams, it's her sister that he'd kill.
Atropos from Insomnia is one of the entities responsible for cutting the life threads of the mortals whose time has come. But unlike the other entities like him, Atropos absolutely despises humans for being such short-lived creatures, and takes great pleasure in ending their lives. So in addition to doing his job, Atropos goes after people whose fate isnít decided and causes them to die horribly, which he has done enough times to fill an entire cave with trophies of his victims. When his boss, the Crimson King, wants him to kill a young boy before he grows up to oppose his plans, Atropos uses this opportunity to harm as many people as possible. He proceeds to corrupt a nice family man called Ed Deepneau into becoming a wife-beating extremist. Under Atropos' manipulation, Deepneau gets together with like-minded maniacs and assault a woman care center, where they murder dozens of people, as a distraction for his real plan: to have Deepneau crash with a plane full of explosives onto a pro-abortion rights rally that the kid will be attending with his mother, killing them along with the other two thousand people present. Along the way, Atropos has several of the protagonistís friends killed just to piss him off, and taunts him about it. Finally, when his plans are foiled, Atropos tries to kill Deepneauís six-year old daughter (who the hero befriended) just to get back at him for humiliating him in front of his boss.
In IT, we have the monstrous entity IT. Though an actual monster, IT is neither animalistic nor alien in its thought processes; the creature has full understanding of human morality, it just doesn't care. In the town of Derry, IT awakens from its hibernation every three decades and proceeds to murder and devour the children of the town, often using the avatar of a Monster Clown named Pennywise to lure children into its clutches. IT prides itself on using its shapeshifting and hallucinogenic powers to torment its victims, preying on their phobias and acquired fears and likening the cultivation of their terror to "salting the meat." From 1740 to 1743, IT was responsible for the disappearance of three hundred Derry Township settlers. In 1957, IT killed Bill's six-year-old brother, George, and devoured Patrick Hoffstetter alive while in the form of his greatest fear, leeches. IT also drove Henry Bowers to madness, then killed Bowers' friends after they succeeded in luring the Losers' Club into the sewers. After waking up in 1984, IT kills a man named Adrian Mellon before resuming its violent killing spree of children. IT proceeds to manipulate Henry Bowers into trying to kill the Losers; drives Bill's wife, Audra, catatonic by exposing her to its deadlights; and manages to kill Eddie before its final defeat.
Leland Gaunt from Needful Things is a very different kind of a monster villain, a genial and well-spoken owner of a little novelty shop that just happens to have your heart's greatest desire in stock. He's willing to sell it to you for a paltry sum and a little favour. This is a harmless prank to play on your neighbour. What you don't know is that your "needful thing" is in fact a malevolent charm that will make you paranoid and obsessed over it and drive you into murderous insanity, and all the harmless little pranks are designed to thrive on old grudges and insecurities, exploit all the dirty secrets your neighbours (and you) have and stir up all the incipient feuds, turning people against each other until your whole town tears itself apart with weapons that Mr. Gaunt helpfully supplies. Even if you're spared by the slaughter and general madness, it is only so you could realise that you're partially responsible for it and kill yourself. Mr. Gaunt will just stand there at the window of his little shop, savouring chaos and death, like he'd done countless times before all across the world.
Rose's husband Norman in Rose Madder has been beating and sexually abusing Rosie for fourteen years, including causing her to miscarry. After he finds out that she left him, he is determined to hunt her down and torture her to death. His methods for locating his ex-wife are tracking down, torturing, and murdering the people who helped her. His favourite method of killing is biting his victims to death. He is also a racist, sexist, homophobic creep who thinks all feminists are lesbians and despises one of his victims as soon as he finds out he's Jewish. His feelings about women are summed up in the fact that when he is in disguise and has to come up with the name of a woman who's protected him, he uses the first and last names of his two favourite porn actresses.
At best, Selectman James P. "Big Jim" Rennie from Under the Dome is a cold-blooded, amoral, greedy, psychopathic bastard. Soon after the start of the novel, we learn that the man is the ringmaster of a massive drug ring, apparently one of the biggest suppliers of meth in the whole country. Then it gets worse. Not only do we learn that he killed his own wife by smothering her with a pillow (a woman already dying of cancer, no less), but over the course of the novel, to ensure that he remains absolute master of the town, he covers up the various murders and rapes committed by his son, Junior, and the gang of thugs he's commissioned as a police force. He kills three or four people (including a pastor, smashing his head in with a gold-covered baseball) who threatened to reveal to his subjects what kind of a monster he is, purposefully causes a riot over supplies just so he can claim need for greater control, and frames the main protagonist, Dale Barbara, for everything that he and his gang has gotten away with. Then, because he had his gang steal huge amounts of propane (the only fuel source in town) just so he could make more meth, he sets the stage for the massive explosion that consumes almost everything in the town, turning the atmosphere into little more than an assortment of poisonous gases, and then gets away with it, hiding away in a fallout shelter. Even worse, he refuses to accept fault for anything that either he did or happened because of his decisions, even so far as to excuse his multiple murders as "sending them into the arms of Jesus," his faith allowing him to dismiss any of the multiple atrocities he does.