These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Works in this franchise with their own YMMV pages:
Complete Monster: The Kurgan is a terrifying Black Knight with Rape, Pillage, and Burn as his way of life, introduced as a brutal warrior who kills The Hero 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 is as brutal as ever who 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.
Cult Classic: The film was a box office flop during its theatrical run (5-some million vs. a $16 million budget), but has gained this status.
Felicia Martins from season 1's "Free Fall" appears at first to be a Damsel in Distress, 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 tries to kill 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 from season 3 is an ancient Immortal who was previously a monk who helped his teacher run a sanctuary on holy ground for Immortals. Kalas, however, used the sanctuary to murder Immortals by ambushing them as they left. After Macleod exposed him and his teacher banished him forever, Kalas vowed revenge. He later attempted to murder a young singer after only suspecting Macleod was involved with her in the 1920s, and when he returned in the 90s, he systematically laid out a web to destroy everyone close to Macleod after murdering his old teacher, Brother Paul. Kalas shows no hesitation in torturing and murdering innocent people, destroying the live of anyone remotely associated with Macleod. At the end, he is willing to reveal the secrets of Immortals at large to world and throw everything into chaos if Macleod doesn't offer his head.
Kern from season 3's "Line Of Fire" is described by his watcher as an "animal," who rapes, kills and steals as he will. Kern in the past 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 had taken. Kern has no remorse in taking an innocent young woman and baby hostage to gain and advantage and happily describes the crimes he has committed over the centuries when confronted.
Ernst Daimler from season 3's "Mortal Sins" 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 from season 5's "Comes A Horseman" and "Revelation 6:8" is an Immortal whose legacy of death dates to the Bronze Age, 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 whom 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 from season 6's "Indiscretions" is unique amongst other 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 showed 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.
Designated Villain: While there are plenty that don't fall into this category, its very easy to make a case that alot of Highlander villains are exactly this. Offscreen, other immortals are almost certainly coming for them, so they come for other immortals. If they can get an extra quickening, their odds of survival go up. A particularly notable case was Tyler King who Attempted to take the head of an idiot savant immortal, knowing that he had absolutely no chance in the game whatsoever, and was thus doomed anyway. To underscore this point after killing King, Duncan and Richie came to the same conclusion anyway.
Riche has it Something fierce with Pete Wilder in Counterfeit.
You can almost hear fangirls around the world squee when he hugs Duncan goodbye at the airport, even more so after the crack about kissing him.
Duncan/Methos, Within ten minutes of meeting the man, Duncan declares he'll protect Methos, just a few hours later Methos tries to sacrifice himself for Duncan. At that point their reasons aren't personal. It's Duncan being the good guy he is and Methos putting the greater good ahead of himself. Pretty soon they start to really care about each other and the chemistry becomes plain as day.
Lord Byron, who came across as being constantly out for threesomes with Methos and whatever poetry groupie crossed his radar.
Fans seemed to think there was something between Methos and Kronos as well, to the point where they asked Kronos' actor about it in a panel.
Moral Event Horizon: Nefetiri in "The Pharaoh's Daughter": She carried a 2000 year grudge against a former Roman General, who was also an immortal, named Constantine. Blaming him for events out of his control (the peace between Egypt and Rome failing and Cleopatra's eventual suicide) she killed his current lover in cold blood since she was his 'entire world'. Infuriated, Duncan kicks her out and she goes to confront Constantine on Holy Ground, forcing Duncan to fight her away from it. When Duncan managed to get the upper hand, offering her one last chance for a new future based on love instead of manipulation? She stabbed him through the heart, literally. Duncan was forced to cut off her head to stop her. If she wasn't stopped, she was well on her way to Complete Monsterdom.
Mako in "Under Color of Authority": A 800 year old lawman, he pursues anyone, no matter of guilt, innocence, or intent with deadly force for the bounty they have. Before the 20th century, this was an acceptable but regrettable practice. In the 20th Century, especially the late 20th century? He comes off as a man who has no real morals and even Duncan calls him out when he says "I am the law!" To get the bounty for a girl who fled Texas after she killed her abusive husband in self defense, he ran her over, and showed absolutely no remorse.