Movie sequels are always a gamble. Sometimes there are sequels worth the wait – always a pleasant surprise – but more often than not, there are disappointing sequels. The biggest no-no, though, are sequels that betray the main character and undo any growth or maturation a beloved hero went through to become said hero. Sequels that undercut characters are worse than those that are pale imitations to their superior predecessors; they erode the careful work of the first film, negating the time and energy taken to craft an incredible character.
Comedies do this a lot, and it's generally more forgivable when they do because they're meant to be taken less seriously (although, there are a couple of exceptionally egregious cases worth noting). When this is done in any other genre, however, what it really does is destroy that suspension of disbelief; it makes the the viewer say, "He'd never do that!" and most of the time, they're right.
Ripley (Sigourney Weaver) almost dies saving the young girl, Newt (Carrie Henn), at the end of Aliens, and for that matter, she almost dies saving Hicks (Michael Biehn) in the beginning. Both Newt and Hicks are great characters, especially Newt, with whom Ripley has almost a mother-daughter relationship just as she needs to see some joy in her life.
That joy is immediately ripped away in the sequel when Newt and Hicks – survivors of so many near-death experiences in the previous movie – die immediately without any lines, and even off screen, in Hicks's case. It's the ultimate undercutting.
Luke Skywalker's (Mark Hamill) last hurrah ticked off a lot of fans, as evidenced by the huge discrepancy in the critics' and audience score of The Last Jedi on Rotten Tomatoes. Many fans believe Luke's rejection of the Jedi ways after Ben (Adam Driver) destroys the temple is inconsistent with the character. Even Mark Hamill, who plays Luke, said "Jedis don't give up," and had no qualms voicing his distaste for his character's reversion during The Last Jedi press circuit.
Luke really strays when he nearly kills Ben in a preemptive attempt to stop the innocent boy. Worst of all, though, is "Hologram Luke," a painfully contrived moment in which Skywalker sends an astral projection of himself across the galaxy to participate in a lackluster climactic battle, which turns out to be his final task before disintegrating in the setting sun. Luke becoming a scared old man in his final days erodes much of the heroism he'd earned in his younger years. Whether or not that's bad storytelling is debatable, but it certainly changed the perception of Luke.
Harry (Jeff Daniels) and Lloyd (Jim Carrey) went from well-intentioned morons in Dumb and Dumber to mean-spirited morons in Dumb and Dumber To. The first movie is a sweet comedy about two guys who just want to belong. The second movie is about a dumb horny dude who wants to get with his dumb friend's daughter.
While their stupidity makes them inadvertently annoying to others in the first movie, they are intentionally assh*les to people in the second, like when Lloyd puts his feet on Harry's parents' dinner table and makes fun of Mrs. Dunne's accent, or when they interrupt a science conference to berate the speaker. In short, the two have become unlikable; they no longer have the redeeming quality of being kind to make up for their profound stupidity.
In RoboCop 2, Omni Consumer Products wants to mass produce RoboCops, but their creations keep committing suicide and they can't figure out why. One doctor speculates that Alex Murphy (Peter Weller), the original, survived the process because of his moral objection to suicide as a Catholic man.
In addition to the fact RoboCop now carries rosaries, he seems to have become more man than machine. He's driven solely by his directives, explaining why he can't participate in the police strike, for example. But the first movie is all about Murphy's humanity winning out in the end, as the film ends with him saying his name. Apparently, that's all out the window. He's all machine now. Any progress he made is gone so they could repeat the process in the sequel.