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Narissa in Enchanted not only has Giselle but Prince Edward out of the way, but instead of blocking their way back to Andalasia, insists upon Giselle's death.
Inverted Trope in the Halloween films. Michael's motives are actually fleshed out in Rob Zombie's reboot. However, it turns out the fans liked it more when Michael was a soulless, mysterious psychopath. This is probably because the "motive" explained in the remake boils down to a cliché Freudian Excuse.
The fifth, seventh and eighth movies are far more psychological in nature and have Pinhead as more of a judge of those whose hedonism and/or ego has made them irredeemable. The sixth and ninth movies largely go back to the original characterization.
The Italian Job (2003): Starts out with a group of thieves making a heist and getting away with $35 million in gold. Then, they're double-crossed by Steve (Edward Norton). They find out where he is and where he's keeping the gold, then they set up an elaborate plan to get the gold back from him. However, as their plans progress, they ignore details that would help them get the gold back quicker. It seems that they're more interested in pulling an elaborate heist than they are with getting the gold back, even after Steve knows they're trying to get the gold back and they know he knows.Justified Trope, because they stated that they wanted revenge from the beginning. They wanted him to know it was they who took everything from him, not just to take it back. They could've just stolen more money from a different source if the money was all they were about.
The Karate Kid 3: Terry. He's supposed to be helping his friend get revenge. The problem is, his friend was really disgraced by his own actions in Karate Kid 1 encouraging his students to bully another kid and to cheat in the tournament (which is the only plausible explanation for why Kreese went bankrupt after one tournament loss) and Terry indicates he's aware of this when he pretends to be a good guy to gain Daniel's trust.
Clyde Shelton had a good start: a complicated Evil Plan to get sweet revenge on the men who brutally murdered his family (one of whom was free after 5 years thanks to the prosecution's bungling). Then he took his revenge further; wanting to kill every official involved in the original trial and eventually all of City Hall, including the Mayor and representatives of other government agencies all in some half-baked bid to up-end the justice system.
It's easy to miss the point of his master plan he's got going. But really, he had snapped to some extent. Once his direct revenge on the two who killed/raped his family was accomplished, his goal was to force people to face the flaws in the justice system (specifically those relating to cutting deals with the guilty and playing politics instead of pursuing justice for victims). It all comes together with his satisfied response to being turned down for a plea bargain at the end of the movie.
Imhotep in these films. In the first film, his every action, from betraying his liege to trying to kill the Female Lead, stems from his commitment to his lover and desire to be with her. That he brings about the end of the world is incidental, the result of being Cursed with Awesome rather than a desire for mass murder or conquest. In the 2nd film, he gains a sudden motivation to challenge the long dead Scorpion King in order to gain control of an undead army and conquer the world, this despite the fact that his love interest is alive and well, together with him, and that he already possesses substantial magical powers. The reasoning is mentioned briefly and never talked about again. The events of the 1st movie stripped him of his immortality forever, so he can be killed by anyone with a gun. The cultists seeking to bring him back want him to kill the Scorpion King so he can use the Army of Anubis to take over the world from a safe distance. It could be explained that since his true love was alive with him and he had immortal, supernatural powers again, they might as well rule the world together, or that while reviving her was his first priority, afterward he wanted to move on to other, less important things. Since we only have his actions and other people's word as to his motives, it's a bit unclear.
At first, Rob Stewart in Revolution 2012 simply wanted to raise awareness of illegal longlining, and the indiscriminate killing of sharks within the marine reserve. However, that changed to trying to raise awareness about the environment around the world.
Doyle works for the defense. He's sent to find out some information about someone on the jury. Near the climax of the movie, his boss (played by Gene Hackman) tells Doyle he needs information before the jury deliberates. Doyle blows him off and continues to investigate, which makes no sense at all. Instead of telling his employer that the jurist is using an assumed name, which would get him thrown off the jury, Doyle continues to investigate as if finding out what the juror's motivation is would be more important than winning the case.
The point of this is that if said juror really is neutral, they can get him to win the case for them. This is quicker and simpler than derailing the court proceedings.
Norman Osborn, a.k.a. the Green Goblin, suffered from this in the first Spider-Man film. At first he wants to off some jerks who are attempting to sell his company, Oscorp. The thing is, after he successfully kills them all, he inexplicably continues to commit seemingly random villainous acts, and is constantly saying things like "Spider-Man is the only one who can stop me," or "Think of what Spider-Man and I could accomplish if we joined forces!"
In the third film, Sandman steals money to buy medical treatment for his sick daughter. He seems to forget about her completely after Spidey foils his first attempt and teams up with Venom for revenge. Instead of, for example, going to another city and/or trying again.
Starman: The Government. The alien's spacecraft was originally shot down under the assumption that it was a Soviet attack, but it's later definitively established that he's an extraterrestrial. NSA Chief George Fox (Richard Jaeckel) still wants to kill him because...well, who knows? Just to be evil, apparently.
Star Trek: Nemesis: Shinzon experiences this within the course of one movie. Initially he wants to free the Remans, and so he takes over the government in order to do this. Then he decides he's going to destroy Earth and offers a throwaway line about refusing to bow to the Federation or any other galactic power like the Remans had done with the Romulans. Also he engages in a bit of Mind Rape with Troi just for the hell of it. Alternately, he was evil all along and freedom for the Remans was simply a ploy to get Picard to trust him.
Star Wars: Anakin Skywalker aka Darth Vader turns to The Dark Side to save his beloved wife from dying. He ends up murdering her in a fit of jealousy/paranoia instead. By the time of the Original Trilogy he is still serving the Emperor and perpetrating galaxywide villainy, apparently for no better reason than that after killing or permanently alienating everyone he ever knew or loved, he has nothing better to do. Incidentally the Trope Namer for The Dark Side Will Make You Forget, but... yeah, that's a bit sloppy. This is why the Jedi have such an aversion to the Dark Side: you gain increased power, but lose control and judgement and wind up destroying what you love.
In this Steven Spielberg film, there is an example with Immigration Officer Frank Dixon. At first his motive is simply to get Viktor Navorski out of his terminal, but when Navorski's immigration problems are finally sorted and he is able to leave to go to New York, Dixon attempts to stop him for no adequately explained explanation.
Pride. In reality this kinda thing doesn't happen this way, they have rules for it, had Viktor simply violated any of the rules, or left the building, he would have been taken into custody and much more quickly had his situation handled because attention was brought to it. By "playing" Dixon's game and adapting to what new rules were being applied (making a new job so Viktor couldn't get quarter returns from carts so he could eat, getting paid under the table for construction, living in a closed terminal, a close friend violating terms of citizenship so Viktor wouldn't be in trouble) he beat Dixon's attempts to get rid of him, and so if he couldn't win, neither would Viktor.
X-Men: Magneto's characterization is particularly inconsistent in these movies: in the first one, he wants to make everyone mutants so that the world can live as brothers (admittedly, he isn't swayed by the fact that the machine will actually kill people instead). By the third movie, Magneto is killing far more mutants than humans — mostly mutants who are willingly following him. To emphasize his shift to Skeletor evil, he not once, but twice announces that "In chess the pawns go first" as his minions die/get depowered pointlessly before him. After the minions fall, he calmly notes "That's why the pawns go first." When Magneto himself is depowered, it is actually serious business. He wasn't a particularly nice guy earlier, but the 3rd movie takes it much further. By this point it should come as no surprise that this motive decay was a result of a change in directors.
Wolverine points out in the first movie that he didn't actually need Rogue at all and if he was really serious about protecting mutants he would sacrifice himself to his machine rather than using her. So this may be a case of his subtlety rather than his motives decaying.
Happens midway through. Wolverine thinks Sabretooth killed his wife and, obviously, wants him dead. When it turns out she's still alive he still wants to kill him... for some reason.
He also murdered Wolverine's only friend, Wraith and beat Wolverine repeatedly, at one point breaking his claws. And he's stated in no uncertain terms that he intends to hunt Wolverine and deny him any happiness, ever. And revealing that the love of your life was a long con just might set a man off.