Complete Monster/Harry Potter

The Books

 * Lord Voldemort. He may be such an archetypal villain that he's harder to hate with the passion fans reserve for Umbridge. Regardless of that, he's still notably an utter sociopath. The very first chapter of the first book is all about how he killed two young parents and tried to kill a one-year-old. He started TWO genocidal wars against Muggle-borns, voluntarily split his own soul into "lucky seven" parts using the murders he committed, and casually uses Crucio, one of the three spells so horrible that one use puts you in Azkaban for life. He uses these spells to a When All You Have Is a Hammer proportion. He gets even worse in the later books, as his magical transformations and soul-mutilations have taken a toll on his mind, driving him pretty much completely Ax Crazy. The sixth and seventh books tried to reason away his transformation into a monster, but the fact remains that he was a heartless, manipulative killer before he even graduated. And rather than feeling any remorse at his actions, his only concern was making sure he could avoid discovery and continue his education.
 * In all fairness to Voldemort (not that he deserves any), it's kinda-sorta implied that he didn't choose to be this way. Specifically, in the sixth book, it's revealed he was conceived under a Love Potion, the implication being that children conceived this way always turn out sociopathic. This could have been mitigated had he been raised by a loving mother, but his mother was so distraught that she allowed herself to die after giving birth. In all, a perfect storm of Freudian Excuses resulting in an utterly evil bastard.
 * If anything, this idea about babies born under Love Potions only reinforces what a monster he is. If there are other sociopaths out there born under the same conditions as Tom Riddle, than why did Tom end up standing out among them and becoming the most feared and reviled wizard of them all? Because he chose to become that! He's heinous even by sociopath standards!
 * Dolores Umbridge is a domineering and abusive matron figure and a bureaucrat whose pettiness and personal failings cause catastrophic harm to those under her control, with a constant housewife-smile about her in the film, even when she's administering those horrible crimes against humanity. She forces misbehaving students to cut up their own skin, which is surpassed by talking herself into both performing an illegal torture spell that has been proven to cause insanity, on a minor to boot. She goes from annoying to so repugnant and wicked that many readers despise her far more than the series' Big Bad. Doesn't even end there. She was also the reason why Remus Lupin, an unwilling werewolf, wasn't even able to find a job elsewhere and the reason why She orchestrated a situation where  And if that's not bad enough, in the seventh book, she presides over trials for the sake of giving innocent children a Fate Worse Than Death, just for being Muggle-Born.   In the book, nothing happens to her; however, JK Rowling said in an interview that she went to Azkaban. In the movie, however, she gets the same fate she prepared for the Muggles.
 * Speaking of the last book, remember Voldemort's Locket!Horcrux? When the heroes (Harry, Hermione, and Ron) wore it they struggled to function, becoming ill-tempered and finding it difficult to conjure any sort of Patronus. Umbridge? She developed an affinity for this object, exponentially amplifying her magical power to the point where she could cast and uphold indefinitely a Patronus so powerful that its mere aura was enough to repel an entire Dementor swarm. Whilst she was also holding a trial. Effortlessly. Harry even alludes that she seemed to be in her element at the time.
 * Bellatrix Lestrange, a psychopathic female Death Eater who seems to be motivated primarily by her desire to inflict as much pain on as many people as possible - except Voldemort, on whom she has an obsessive crush. Bellatrix's insanity makes her an unfettered case of The Fundamentalist, and while Voldemort is a cold and unfeeling murderer, Bellatrix is gleeful cruelty incarnate. She has a special fondness for torturing people with the Cruciatus curse, sometimes to the point of driving them mad from the pain. Actually, the depictions of her passion at points would almost produce some empathy for her. But that emotion is so horribly misguided that it only emphasizes her monstrosity. Bella is actually given one humanizing quality: she genuinely seems to care about her younger sister Narcissa. However, given that she makes a point of personally executing relatives who stray from the family tradition, it's quite likely that this would last only as long as Narcissa is loyal to Voldemort. . Bellatrix is so masterfully characterized that she's legitimately the most terrifying character in the whole story. Her Karmic Death at the hands of was probably the most satisfying moment in the series.
 * Fenrir Greyback. It's one thing for a werewolf to go helplessly violent in wolf form; it's another to bite people in human form just for the pleasure of ruining their lives. Or purposefully hiding in the woods near a family's home on the night of a full moon, so that when he transforms, they'll be his first targets. Not to mention that his obsession with biting children has disturbingly pedophilic overtones and he's heavily implied to engage in full-blown cannibalism too. According to implications, he wanted to rape Hermione when the main trio were held captive in Malfoy's manor.
 * He'll kill you, rape you, and eat you. And if you're really lucky, he'll do it in THAT order. And he also did have hygiene standards below those of a true Canis lupus. Dogs and wolves do groom themselves up to a point. Well demonstrated in the film adaptation to the Deathly Hallows, in which Fenrir is shown  during the Battle of Hogwarts. Notably, he was in human form at the time, underscoring that his behavior isn't attributable at all to his being a werewolf.
 * Fortunately, this is also his last scene, as Hermione gives him exactly what he deserves.
 * Had they done more on-page and on-screen, the Carrows would be this too. They were the ones who forced students to practice the Cruciatus Curse on first-years, though, so they pretty clearly fall into Complete Monster territory. Neville even stated that they made Umbridge look tame by comparison, which makes the readers really speculate how horrible they could be!

The Film Series

 * While he has a Freudian Excuse in the book - that of a "Well Done, Son" Guy who found approval in Lord Voldemort - and seemed pitiable in the face of his abusive father - is this in the film version of Harry Potter and The Goblet of Fire.   While all this occurs in the book as well, he's presented without any sympathetic traits here, and even his motive has seemingly become For the Evulz (he even has something of a signature tongue flick resembling a snake!).