Mushu from Mulan II. It was all just because the ancestors did the same, threatening to separate him from Mulan, and he didn't want that.
In Cinderella III: A Twist in Time, Lady Tremaine. She was already a jerkass, but now she goes from abusing her stepdaughter to trying to kill her. Inverted with Anastasia, who becomes sympathetic and eventually makes a Heel-Face Turn.
Clopin from The Hunchback of Notre Dame is first introduced as the colorful narrator of the film, and the master of ceremonies at the Feast of Fools, where he was very kind to Quasimodo. However, near the end of the movie, he is now the bitter judge (his hand puppet serving as Quasi's defense) presiding over the Court of Miracles, and is now very mean to not only Quasimodo, but also Phoebus as well, and as a result he wanted the two both sentenced to death for being spies of Frollo (they were actually working against Frollo, not for him). Had Esmeralda not showed up at the Court of Miracles to stop Clopin and tell him that both Quasi and Phoebus are trying to help them, Clopin would have actually killed them both.
This seems to be more about Hidden Depths than becoming more of a jerk - Clopin's mostly rather genial on the outside, but he's also secretly the leader of an insurgence desperate to stop a madman's oppression of his people (so he's a bit of a Stepford Smiler). He was likely exercising caution to protect his people and simply acted hastily - though he could have at least waited for their side before passing immediate judgement, true, it's unlikely he would have believed them without Esmeralda vouching for them, and one must remember that Phoebus was the Captain of the Guard prior to this event (information which had likely not yet reached Clopin), so yeah...
Harry Osborn. Goes from a mostly nice guy in the first movie to lashing out at Peter in the second movie and acting like a douchebag for most of his scenes. In the third film he gets better via a little Easy Amnesia, but then reverts to full blown Rival Turned Evil. In the end, he redeems himself, but Redemption Equals Death.
Mary Jane also becomes increasingly self-centered as the series progresses. She has No Sympathy for Peter despite all the problems he has, refuses to hear his side of the story as to why he missed her play, and then dumps her fiancee at the altar to get with Peter! In the third movie she resents Spider-Man's popularity for saving people's lives and not having the time for her, but this time she at least recognizes this as negative behavior, does feel bad about it and tries to do better....but life doesn't make that easy for her.
Even Peter himself took a level in the third movie. He develops a little Acquired Situational Narcissism in the beginning, then got on a vengeance kick, and then he got the Venom symbiote. You know the rest.
Titanic has the obnoxious aristocrat Caledon "Cal" Hockley, who starts out as a pompous, misogynistic Jerkass. He soon graduates to smack his fiancee around, trying to frame Jack for burglary, and when things really start to go south, kidnapping and attempted murder.
Ash of the Evil Dead series does this over time. In the first film he's basically a Final Guy, by the second he's a Bad Ass, and in the third he's a full blown Jerk Ass. It makes sense, though. After going through some horrific events, losing a limb, nearly losing his mind, and having everyone die on him, he gets sucked into a time period where its residents initially treat him like crap, make him a slave, and then try to kill him. It's probably safe to say it'd be more disturbing if he didn't have a huge chip on his shoulder by that point.
Abigail Chase from the National Treasure series seems to take one between the first and second movies. In the first movie, she starts out as a tad bitchy, but really not without reason given the crap she ends up in thanks to Ben. By the end of the movie, she's considerably nicer and has evolved into a proper Love Interest. When the second movie comes around, she's reverted to the beginning of the first movie and gone even further, being worse than she was then. She changed the alarm code in what is most likely Ben's house, and she's hostile around him even when he's trying to be nice. He also has to bribe her to get her to help him clear his family's name.
Halloween II (2009) had Loomis of all people go through this, having no problems with morality or ethics whatsoever when it came to writing a book that allowed him to profit off the tragedy of the previous film. Fans were not happy about this change.
In Iron Man 2 Tony Stark allows the fame to get to his head again, and it screws him over worse than his playboy tendencies did in the first film. However, it's something of a ruse. The power source for his arc reactor is poisoning him, pretty much having months to live/find an alt power source. With Hammer trying to get his tech and all of this, he lashes out yet also does it in a matter so Pepper can run the company and make sure it remains right while goading his friend into taking the prototype armor to become War Machine. He doesn't fully recover after seeing the final messege from his dad, getting some much needed faith and hope.
In Scarface (1983), Tony Montana starts out the film as a crook, but it's not until he gains power that he allows himself to become a cocaine-addicted wreck. He constantly curses and acts like a cantankerous prick towards even his friends, and gets called out on it by almost everyone close to him.
John Rambo in the fourth movie - although he is still one of "the good guys" - he is portrayed as more rude and cynical than in the first three movies. Considering everything he's been through up 'til then, it's perfectly understandable that he'd be a left a little bitter.