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.
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.
Kane from Highlander: The Final Dimension 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 to him.
Idiot Plot: The Quickening. There was clearly no plan in place for the setting's wider mythology when the first film was made, so the reveal that the Immortals are aliens was just randomly thrown out, opening up quite a few plot holes. Why do they care about human holy ground? Who the hell would want to go back to Zeist when Earth is an infinitely better place to live?
Speaking of Blake, he was surprisingly close enough, while at first scared of Katana, to being a Complete Monster to stand his own a bit with Katana, even his plan to use to a descending spinning fan on the three heroes manages to be able to kill Ramiez in the process! Too bad he made the mistake of telling Katana that he "fucked up" afterwards, resulting in getting him getting grabbed by the balls by the Immortal and thrown out a big window.
Romantic Plot Tumor: None of the films have particularly well developed romantic subplots, not even the first one. The sex scenes on the other hand...
Sequelitis: One of the most infamous instances of it.
Special Effect Failure: Connor's makeup in Highlander: Endgame. Just watching the movie, you think that Christopher Lambert is horribly showing his age, and it's impossible to see him as the Immortal 18-year old Connor Macleod. Until you watch the cast interviews and see that Lambert is actually not all that bad-looking, and that they had to have made him look 70 intentionally. Presumably it's to reflect how he's tired of life and ready to give up, but without any explanation, you just assume it's the actor too old for the role.
When Connor receives the prize in the first movie, the wire holding him up is visible in multiple shots.
Freddie uses his iconic mic stand, and battles Connor to a draw. What does that tell you?
What an Idiot: While The Quickening was revolting, one scene made it worth the watch. Conner's "The Reason You Suck" Speech to General Katana for trying to kill him, when he already was planning on dying of old age.
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.