Lord Voldemort. His defining characteristic is being a sociopath who cannot understand love and has no remorse for any of his actions. His Establishing Character Moment is killing two young wizarding parents and his attempted murder of their one-year-old. He started two genocidal wars against Muggle-born wizards, Muggles, and their supporters alike, out of a Nazi-esque desire to create the "perfect" wizarding world. Most heinously, he is well aware that using the spell to murder other humans will split his soul irrevocably, because of how evil the spell is - and uses this knowledge to create immortality for himself. 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 explain his transformation into a monster, but serve more as a cautionary tale than a true Freudian Excuse: his mother basically enchanted and raped his father, and when his father was freed of the enchantment, he bolted, and his mother died of despair. This led to him growing up without any idea of love or what it entailed.
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 in the name of Voldemort's cause. 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, and tortured Neville Longbottom's parents to the point where they exist in near-comas, and will never recognize their own son again. She is also an accomplished arsonist, burning down multiple residences basically just for the sake of it. She's the cause of death for half of her non-Death-Eater family members, including fan favorites Sirius Black and Nymphadora Tonks, and delights in the torment these deaths cause. Common readings of her also note that she is obsessed with Voldemort, and relishes his cause more than any other Death Eater in the series. She receives a Karmic Death at the hands of Molly Weasley when she threatens Ginny during the assault on Hogwarts.
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. His actions are all-but-explicitly stated to be pedophilic in nature, based on what Fenrir did to Lupin and his desire to rape Hermione after Bellatrix tortures her, and to top it all off, he's a cannibal. While this is merely implied in the books, it's flat-out shown in Deathly Hallows Part Two. He shows absolutely no remorse for doing any of these things, and takes a perverse delight in the damage he causes. The only reason he showed up at the Battle of Hogwarts was to engage in raping and eating children, after all.