Complete Monster/The Vampire Diaries

The Vampire Diaries and its Darker and Edgier Spinoff, The Originals, operate on Black and Gray Morality, with very few characters truly pure. A select few, however, are completely and irredeemably evil.

The Vampire Diaries
Three villains on The Vampire Diaries were particularly monstrous:
 * Markos from Season 5 is the leader of the Travelers, a subculture of witches restrained from using magic. Markos planned on undoing that spell along with every spell ever cast by a witch, viewing their magic as an abomination and his own as pure. Centuries ago, he enacted a curse so that certain generations of the descendants of Silas and Amara would be their exact Doppelgangers, brainwashed into being drawn together, so he could use their blood for the Travelers' ritual; even before he is resurrected, he orders the death of Stefan's innocent doppelganger, just so Stefan and Elena will be the only ones left. He allows his most loyal minion, Sloane to die just to demonstrate the result of his curse; begins the destruction of the Other Side, an afterlife for supernatural creatures, causing its denizens to start hurtling into the void, resulting in the sacrifice of his own minions; later abducts and tortures Stefan and Elena for more of their blood; leads the Travelers in a mass ritual to possess the population of Mystic Falls, destroying their original bodies to make the change permanent; and tries to unleash an anti-magic field that would have killed every vampire on the planet. In the season finale, Markos has Tyler/Julian killed, after learning that Julian killed Stefan when Stefan was still useful to them. When Markos suspects a plot to kill him, he keeps Caroline's mother Sheriff Forbes close to him, and pins her down in his dying moments. While most villains on this show have honorable or sympathetic qualities, Markos only cares about achieving his goal, by any means necessary.
 * Julian from Season 7 stands out as an especially evil vampire in this series, constantly spending his time murdering and manipulating. Julian is Lily Salvatore's lover and father figure to the Heretics, whom he sees as personal pawns. In his first episode Julian discovers that Valerie is pregnant with Stefan's baby and beats her into a miscarriage, silently taunting her as she recovers. When resurrected, he compels multiple people from other towns into filling up the school and restaurants as a constant and living blood bank. To protect himself, Julian links his life to Lily's. When Lily sees through him, Julian tries to force Lily to either kill Damon or Valerie, taunting that he unlinked their lives when Lily stakes herself. Julian later uses a mystical dagger to entrap Stefan and Damon in their personal hells. Julian also kills off mall employees at a neighboring town. When Stefan and Damon return, it's discovered that Julian has built an chaotic vampire army that has completely taken over Mystic Falls, which Julian uses to feed off and torment the human population. Julian also holds a gladiatorial fight club where contestants fight to the death for his amusement, in which he has a suicidal Damon take part in, before fighting Damon himself. It's also revealed he's behind the vampire hunter Raina, having forced the girl to kill her father. Julian doesn't even have the humanity switch to excuse him, being outright stated to have been born evil and having no interest in turning even attempting to turn to goodness.
 * Arcadius, aka Cade from Season 8 was the world's first psychic and was once loved and respected by his people, until they betrayed and executed him, which lead to Cade developing a hatred for humanity, and in his last moments used his power to create a Hellish dimension for him to rule over and feed on souls. After taking his revenge, Cade would corrupt Sybil and Seline into the cannibalistic sirens, leading to countless deaths, corruption and damnation over the centuries. Cade will also take the souls regardless of how small their sin was, if they were remorseful or otherwise good people. It's also discovered that Cade was the force dragging the denizens of the Other Side into Hell upon its destruction. When summoned to Earth, Cade is entertained by the notion of warping innocent three-year-olds Josie and Lizzie into his new servants, before Damon and Stefan offer themselves in their stead; Cade reveals to Stefan that he wants him specifically for his work as the Ripper and wants him to seek out innocent souls for him to claim. When an attempt to sacrifice Mystic Falls fails, Cade is summoned and has Seline and Sybil burned alive. When Damon regains his humanity, Cade tries to force Damon to kill a hundred strangers or kill Caroline as a sacrifice, before revealing he sent Stefan to track down and kill Elena; later, Cade uses Elena as a bargaining chip to find the weapon that can kill him and, in the final fight against him, tries to force Damon to choose between Stefan and Elena's soul before trying to claim Damon himself. Initially presented as a tragic and noble figure, Cade is not only aware he became the same pure evil he supposedly sought to eradicate, but is proud of it.

The Originals
The Originals, spinoff show of the above, gives us these examples:
 * Monique Deveraux from Season 1 stands out and proves that evil knows no age. A teenage girl killed in a botched Harvest Ritual, her aunt Sophie struggled to have Monique resurrected, and when she succeeds, Monique repays her aunt by murdering her and seeing it as another casualty of war. She would later go on to assist the witch leader Celeste, and after Celeste dies, does the same to their new leader Genevieve. Hardened by her religious loyalty to the Ancestors, Monique is willing to kill Genevieve, simply because they told her to, only relenting when they order her to kill Klaus and Hayley's unborn baby, and sadistically attempting to kill Hayley to induce a miscarriage. In addition to this, Monique scapegoats Davina, blaming her for the Harvest going wrong, simply because she survived and gets the other witches to do the same. When Hayley goes into labor, she and the other witches abduct her; Monique tells her that they will kill her baby and there is nothing she and Klaus can do about it. After Hayley gives birth, Monique slits her throat and leaves her to die While preparing the ritual to kill the baby, Monique talks to another Harvest girl about usurping Genevieve all together. When facing defeat, Monique attempts to stab the baby. While she does have a Freudian Excuse in being killed in a ritual and corrupted by the Ancestors, the other Harvest girls went through the same thing and didn't come out as evil or fanatical as Monique, who willingly reaches depths that most vampires, witches and werewolves in the conflict don't.
 * Francesca Correa from Season 1 is a descendant of the thought to be extinct Guerra werewolf clan/crime family, but poses as a human to control their faction. Despite signing a peace treaty between werewolves, vampires, witches and humans, she helps intensify the conflict by masterminding the bombings on the home of the Crescent werewolves, with children in the home, in order to frame the vampires. She intimidates Father Kieran's niece Camille into looking for a key that might expose her family for what they are. Francesca is revealed to be in cahoots with the witches, and oversees the witch leader Genevieve, who makes the moonlight rings. In return for the rings, Francesca gives the witches Hayley so that they can kill her baby. With these rings, Francesca and her brothers massacre a small army of vampires and savagely maul Elijah. Francesca cares not for her family and minions, using even her brothers as bodyguards, claiming that she is the top dog and that it's her minions job to keep the fight from getting to her. She maintains control by running her werewolves as a gang, all while secluding herself to avoid retaliation. When the fight does go to her, she flees while Elijah kills her brothers.
 * Lucien Castle from Season 3, one of the first vampires turned by the Mikaelsons, is defined by his arrogance, sadism, and ambition. Originally a timid peasant and servant, over time Lucien would evolve into a dark reflection of Klaus, later exceeding him in evil. The CEO of Kingmaker Land Development Inc., Lucien uses his position to hire hunters to wipe out the werewolves in Louisiana, with the few surviving werewolves taken by his company to be experimented, on along with vampires, for his further ambitions. The prime suspect of serial killings in New Orleans, Lucien would frame Camille for the murders, before psychologically torturing her and Detective Will Kinney for information. Briefly acting as an uneasy ally to the Mikaelsons, Lucien betrays them after Klaus is unlinked to his bloodline. Lucien reveals he made a deal with the aforementioned Ancestors to help take over New Orleans and kill the Originals. In return, Lucien would gain more power and rule over the remaining vampires. Upon his transformation, he kills Finn. Lucien attempts to kill Elijah and his bloodline, just to kill his rival Tristan. Upon discovering Aurora doesn't love him, Lucien denounces his previous love for her, stating love is a weakness and declares he will kill everyone Klaus loves. Lucien then kills Camille, and, just to hurt Klaus, plots to kill Rebekah, not caring that Aurora is part of her bloodline. His last action is to force Klaus to kneel to him, threatening to rip out Hayley's heart. Despite his Freudian Excuse in being looked down upon by nobles and tortured by Tristan, he openly denounces any valid excuse he may have. As he grows in power, he is no different than the nobles who mistreated him, looking down and tormenting those he sees as beneath him.
 * Inadu, aka the Hollow, the Big Bad of season 4, is an ancient witch who seeks power and domination over all things. In her mortal life, Inadu was bestowed great powers since birth and desired more, with a life time of mass murder and chaos purely for the fun of it. When she was first killed, she casts a final curse on her people, creating the curse of lycanthropy in the process. As the Hollow, over the centuries she had cultists who followed and worshiped her, carrying out dark deeds, mostly child sacrifice, to her name; she was also behind several serial killers. She drives her followers into insanity and even suicide, including Evie Sinclair and Jackson's grandfather, having the latter kill Hayley's parents. Following the downfall of the Mikaelsons, the Hollow's followers begin a new spree of sacrifices for a takeover of their own. Upon the Mikaelsons' return, the Hollow tries to draw power from Klaus and Marcel, and tries to goad them into killing each other; the Hollow threatens to trick Klaus into killing Hope via hallucinations. When the Ancestors return, Inadu hides by possessing Sofya, and seeks her bones in order for a full resurrection. Later she abducts Elijah to slowly kill him and his bloodline in a ritual sacrifice. Following her resurrection, she rewards her followers by killing them, and resurrects Davina, linking their lives in order to blackmail Kol. Later she torments Hayley and Freya with hallucinations. Despite seemingly being killed by Hayley, she proceeds to possess Hope, attempting to completely take her over. Finally, Inadu uses Hope's body and power to lead her followers on to take over New Orleans via conquest, wrecking havoc and threatening to kill those who won't follow her.