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 from Highlander 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.
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.
I Am Not Shazam: There are times when the Immortals as a whole were called "Highlanders" by viewers new to the franchise.
Freddie uses his iconic mic stand, and battles Connor to a draw. What does that tell you?
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.
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.