Complete Monster/Harry Potter

The Book Series

 * 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 then tried to kill their one-year-old. He started TWO genocidal wars against Muggle-borns, voluntarily split his own soul into seven parts using the murders he committed, and casually uses Crucio - one of the three spells so horrible that a single use puts you in Azkaban for life, and it's practically his bread and butter proportion. Naturally, he would only became worse during the later books in the series: his magical transformations and soul-mutilations have visibly taken a toll on his mind, driving him pretty much completely Ax Crazy. This manifests in a compulsion to kill even when it's completely unnecessary; during a flashback to the night of the murder of Harry's parents (that Harry himself experienced from Voldemort's POV), Voldemort encountered a child who mistook his face for a mask. Said child fled when he quickly learned that wasn't the case, and yet he still contemplated killing the child for the "grievous" offense of basically looking at him funny. And yet, for all his craving of death and destruction, it also proved to be his biggest fear - so much that he was willing to utterly destroy his soul and the lives of many innocents in order to escape death and rule the Wizarding World.
 * The sixth and seventh books explain his transformation into a monster, but do not attempt to change the fact that he was a heartless, manipulative killer before he even graduated. Rather than feeling any remorse for his actions, his only concern was making sure he could avoid discovery and continue his education. Specifically, in the sixth book it's revealed he was conceived under a Love Potion, with the implication 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. To further the point, 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 in a kangaroo court for the sake of giving innocent Muggle-born witches and wizards a Fate Worse Than Death by shipping them off to Azkaban where more than a few of them died, just for their non-magical lineage and threatens to allow the Dementors to perform the Dementor's Kiss should they try to resist. And to top it off, these trials make her disturbingly similar to real-life Nazi war criminals.  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. If there's anyone who deserves the Dementor's Kiss, it's definitely her. And hilariously enough? It turns out that her intense hatred for Half-Bloods and half-breeds comes from the shame of being a Half-Blood herself, making her not just a loathsome bigot, but a hypocritical one as well.
 * 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. In fact, Rowling herself explained that Umbridge is such a horrible person, the Horcrux's aura was aiding her rather than harming her, hence how she was able to maintain such a powerful Patronus during a trial.
 * Not to mention the fact that Umbridge was working for a regime Voldemort was controlling from behind the scenes. By this point in the story, the Ministry of Magic had acknowledged Voldemort's return, were trying to stop him, and the previous Minister of Magic had been assassinated and the Ministry of Magic was under Death Eater control. Rather than fight back against the coup, Umbridge signed on with them. She may not have gotten the Dark Mark and become a full Death Eater, but she's already worse than most who have.
 * Bellatrix Lestrange is 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. In fact, she was the one primarily responsible for torturing Neville Longbottom's beloved auror parents into insanity and is disgustingly proud of it to the point to where she casually taunts Neville about it when they first meet. 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 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 such as, it's quite clear that this would last only as long as Narcissa is loyal to Voldemort. Bellatrix even said in the fifth book when they're talking to that if she had any sons, she'd gladly give them up in service to Voldemort. . Bellatrix is so masterfully characterized that she's legitimately the most terrifying character in the whole story with her torture of Hermione (ESPECIALLY in the movie, which added some creepy, creepy rapey overtones to it) being one of the most nightmarish parts of the entire series. 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 and turning them against wizarding society has disturbingly pedophilic overtones. And let's not forget that he's the reason Remus Lupin is a werewolf in the first place, shall we? Even worse, he bit Remus just because his father insulted him. Even though Malfoy . Fortunately, in his last appearance in the books he gets taken down and his last scene in the film has Hermione gives him exactly what he deserves.
 * On top of that he even engaged in full-blown cannibalism too. When Dumbledore saw him in the sixth book, he voiced his suspicion that Greyback had a developed a taste for human flesh even when not in werewolf form. Greyback not only proudly confirmed it, but spoke with relish of "...throats to be ripped out... delicious, delicious" then offered to kill and/or eat Dumbledore afterwards. In the seventh book he outright wanted Hermione for himself when the main trio were held captive in Malfoy's manor and was very keen on the idea.
 * His desire for Hermione in the books came across as sexual (he even taunted Ron about it), and given his tastes he could've wanted to rape her, eat her or both. Essentially he'll rape you, kill you, and eat you. And if you're really, really lucky, he'll do it in THAT order. Not to mention in his last appearance he went to eat Lavender Brown during the Battle for Hogwarts in the book, and was found gnawing on her bloodied neck in the film.
 * Marvolo Gaunt, the grandfather of Lord Voldemort himself, is actually one of the most disturbing examples of a complete monster in the entire series. As a result of a centuries-long incestuous tradition in his family, Marvolo turned out to be a deranged and psychopathic man capable of shocking acts of violence and abuse. He is an ancestry-obsessed narcissist who sees himself as utterly above everyone and everything else around him solely because he is descended from Salazar Slytherin himself, as well as the Peverell family. Anyone who disagrees with him, he openly declares to be a Mudblood - a label he holds in the uttermost contempt possible - despite not previously knowing that person's true blood purity. His treatment of his children is even more of a testament to his horribleness: His son Morfin is a brutish, bloodthirsty and barbaric man whose affinity with snakes reaches disturbing lengths, and Marvolo sees any violent act by him against Muggles as completely justified, being personally insulted by the suggestion that they aren't. In general, Marvolo sees Muggles as pests to be trodden on by greater men like him. As for his daughter...oh dear lord, where to begin? He treats her like nothing short of a slave (think the way the Malfoys treated Dobby, times it by a few dozen, and then make Dobby a human woman who is directly descended from the family she's serving, and you've got Merope's treatment by her family). He thoughtlessly carries out frequent magical, physical and verbal assault on Merope for the slightest things, even if she wasn't doing anything to offend him and just happened to be in the room with him at the time.

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!
 * One of the things that makes him especially loathsome is that, along with Bellatrix, he helped turn Frank and Alice Longbottom insane. To be so proud of such a disgusting action that you show it to the nervous woobielicious son of your torture victims shows that  is even more of a bastard than your standard Death Eater.