Complete Monster/Highlander

Highlander has many of these, so contrary to the famed tagline, there is NOT "only one":


 * The Kurgan, a terrifying, Ax Crazy, combat-loving Blood Knight with Rape, Pillage and Burn as his way of life, is introduced as a brutal warrior who kills Connor MacLeod in battle. When he hunts down the now-immortal Connor, the Kurgan takes the head of Connor's mentor Ramirez and, believing Connor's wife Heather is Ramirez's lover, rapes her to cement his victory over Ramirez. Centuries later, the Kurgan, now going by "Victor Krueger," as brutal as ever, has no compunction killing innocents in his drive to be the last Immortal standing in The Game and mocks Connor about the possibility that Heather enjoyed the rape when he learns she was Connor's wife.
 * Kane from Highlander: The Final Dimension and Highlander III: The Sorcerer is a truly vicious Asian warrior who kills and rapes for fun. He's introduced butchering an entire village with his two immortal companions after the villagers don't give him the information he wants fast enough. He kills Connor Macleod's latest mentor, the wise Japanese sorcerer Nakano, to steal his magical powers. Upon his release in the present after being buried in a cave for centuries, he promptly kills one of his associates without incident after sending the other one off to do his dirty work for him. He continues his rampage by raping a prostitute, promises Connor that he will rape his love interest after he's taken his head, and kidnaps and threatens his son to get Connor to come.
 * Xavier St. Cloud is a particularly nasty Immortal and one of Duncan MacLeod's oldest and most reviled enemies. A Moroccan warrior who perished in the First Crusade and awakened to Immortality, Xavier was found and trained by Henri St. Cloud, an immortal crusader. Xavier would later show his gratitude by taking the name St. Cloud when he murdered Henri for his first Quickening. Xavier would become a vicious headhunter for centuries, and when he took his own student, helped the young man gain revenge on his family for rejecting him by murdering them. Later dedicating himself to hedonism and wealth, Xavier took to gassing soldiers in World War I to rob the corpses, where he would encounter Duncan MacLeod again and nearly killed him when he was weak. In the modern day, Xavier robs jewelry stores by gassing everyone inside, including his mortal accomplices (whom he told the gas was just sleeping gas, at which point they walked inside and died; he did this just for fun. He would then give a sham confession to Immortal priest Darius simply to mentally torment the kind-hearted priest. After losing a hand to MacLeod, Darius would team up with the Immortal-hating Hunters with a bargain that revealed the depths of his lack of honor: they would gun down Immortals while Xavier took their heads. Arrogant, selfish, cruel and wicked, Xavier represents the worst excesses of Immortality.
 * Early villain Felicia Martins appears at first to be a Distressed Damsel, a young Immortal on run from the run from the brutal, obsessed Claude Deveraux. It is revealed that Martins is far older than she claims and Deveraux is hunting her because Martins murdered his wife and their adopted baby daughter. This is her MO: she ingratiates herself into the lives of her victims and murders everything they love, making it so they're off their game and she can easily take their heads. She attempts to murder hero Duncan Macleod's loved ones after she takes Deveraux's head and is last seen vowing to return and murder who Duncan loves after he spares her.
 * Kalas, the Big Bad of Season 3. Kalas used a sanctuary for immortals as a trap to hunt them down and kill them as they exited. When Duncan MacLeod exposed him and had him exiled, Kalas retaliates, centuries later, by trying to strangle a girl Duncan might be having a fling with. In the present, Kalas is a ruthless manipulator and murderer. He can destroy someone's life easily and shows no hesitation in torturing and murdering innocent people, and no remorse for those he uses as pawns in his game to hurt MacLeod. At the end, he's even willing to reveal the secrets of Immortals at large to world and throw everything into chaos if MacLeod doesn't offer his head.
 * Kern of "Line of Fire" is described by his watcher as an "animal," who rapes, kills and steals as he will. In the past, he was a scout for American forces who loves killing Native Americans. He earned MacLeod's enmity when he helped slaughter the Sioux village MacLeod was living with and presented Duncan with the scalps he'd taken. Kern has no remorse in taking an innocent young woman and baby hostage to gain an advantage and happily describes the crimes he's committed over centuries when confronted.
 * Ernst Daimler is the only Immortal seen who's a former Nazi. A firm believer in fascism, Daimler sought to help 'purge' all lower races in World War 2 and ordered the slaughter of hundreds of innocent people, as well as wounded prisoners, until he was stopped by a young boy who stabbed him in the back. Thinking he was dead, the boy and his cousin wrapped Daimler in chains and threw him into the Seine where he remained for decades. In the present, Daimler leads a white supremacist movement and finds the little boy who stabbed him, now a kindly old priest. Daimler gleefully guns him down after terrifying him off holy ground, and then attempts to murder a pregnant woman who tried to stop him before Macleod intervenes and takes Daimler's head.
 * Kronos, an Immortal whose legacy of death dates to the Bronze Age. Kronos was the leader of the Four Horsemen, a band of Immortals who led armies to Rape, Pillage and Burn across continents. Kronos was the most sadistic of all of them, slaughtering the innocent and raping who he desired, including a slave girl for no other reason than his sworn brother Methos had taken a liking to her. In the present, Kronos recruits Methos, now The Atoner, back into a scheme to reunite the Horsemen and create a plague to destroy all of humanity, viewing himself as the end of time itself.
 * Morgan Walker is unique among Highlander villains in that he's been a slaver for centuries. While Walker once dealt in the Atlantic Slave Trade, in modern times he uses his job as a modeling agent to capture young women for the sex trade. In prior years, Walker had an attachment to a slave named Charlotte, but after he suspected she had an affair, he killed her throwing her out of a high window and displays no remorse for her death. When hunting Methos, the immortal she had the affair with, Walker also kidnaps the daughter of Methos's good friend with full intention to kill or sell her should her father not betray Methos.
 * In Highlander: The Animated Series, Lord Kortan, refusing to take the pledge the other Immortals took After the End, declared himself the de facto winner of the Game, and decreed that he was now entitled to all of the world's knowledge and power. After killing Connor MacLeod - the only Immortal willing to break the oath to challenge him - Kortan conquered and enslaved most of the planet, ruling unopposed for 700 years. When Quentin MacLeod's immortality activated after Kortan's slavers murdered him and his mother, Kortan set out to slay this new Highlander through any means necessary. Kortan's worst acts included firing an atomic bomb at Quentin's homeland; feeding dissidents to mutant ants; lobotomizing people to make disposable "human keys" to his armory; regularly executing his own followers for failure and outliving their usefulness; deploying them as suicide bombers against a city; and inciting hatred of Quentin by founding a cult in his name, one that enslaved kidnapped children after staging their sacrifice. In the final episode, Kortan destroyed whole towns with salvaged missiles, and threatened further bombardments unless Quentin confronted him at midnight. Even though the deadline was met, Kortan ordered that more rockets be launched anyway, laughing, "Why stop at MacLeod when we can crush them all?"