Avengers Arena: Fans of the characters before Arena find many of their actions within to fit this bill. Primarily, it's either turning characters with deeper complexities into shallow caricatures of their former selves or reverting them to older characterizations.
Mettle is defined entirely by his relationship to Hazmat. Worse, he engages in an action which goes against the sacrifice he makes in the "Final Exam" arc of Avengers Academy.
Juston's prior characterization is jettisoned entirely so he can be defined by depression and revenge. This characterization is not without reason, but there is nothing else about his character that shows up.
Nico Minoru, a mothering character, never mentions the people whom she cares for, even while she is dying.
Chase Stein acts more like he did in the last arc of Brian K. Vaughan's run rather than showing the development and maturity he gained under Joss Whedon.
Cammi is basically a new character, but the progression is logical from where she left off at least.
Reptil is supposed to be the leader of Avengers Academy and has properly led them on several occasions. In Arena, this is not in evidence and in some scenes he seems to cede leadership duties to Hazmat.
X23 has a major Out-of-Character Moment even within the context of Arena itself when she attacks Apex head on, despite knowing the latter is in control of a Sentinel. This is after the series firmly establishes that Laura is constantly determining multiple means of killing everyone around her, choosing the best course of action while discarding those with undue risk.
Black Panther. Poor T'Challa. Opinion is divided about when, exactly (Johns, Hudlin, or Liss) the derailment occurred, but not if. The fact is, in the early 2000s, Panther was treated as one of the smartest men in comics, king of The Plan, and spent his time matching wits with Mephisto, Doctor Doom, Iron Man, etc. and coming out on top. Fast forward to the present, where he and Storm have been Strangled by the Red String in the span of basically one story arc, he's lost his kingdom (due to latching onto an uncharacteristic Idiot Ball), and his IQ appears to have dropped about fifty points. His well-documented scientific skills and background have been downplayed if not out-and-out retconned, and he's gone from being "Batman, only better" to "Daredevil, only worse, and have we mentioned lately that he's black?". His villains have gone from major threats like Doom, Magneto, and Mephisto to never before seen Russian mobsters, and Hatemonger. In terms of standing, he's gone from one of the most respected and revered heroes in the Marvel Universe to being talked down to by Luke Cage on a nearly monthly basis. This direction has been hemorrhaging sales as a result, and with Christopher Priest retired, it fell to Jonathan Hickman to fix the character in his Fantastic Four and New Avengers runs, showing that T'Challa still has a keen scientific mind, and is still respected.
Brian Michael Bendis has a tendency to be horribly tone-deaf on some characters; in fact one of the biggest criticisms about him and his run on The Avengers is that he has no idea of how to write classic big gun characters like Captain America and The Mighty Thor, which might explain why so many of them were forced out and replaced with the kind of snarky, street level characters he tends to write better.
One example is Dr. Doom — ruler of Latveria, world-class evil genius, and refined bastard — telling Carol Danvers to "shut [her] cow-mouth" or else he'll stop her "whore's heart".
In a one-shot made to set up Mighty Avengers, Jarvis refers to Tigra as a "b@#ch", something Jarvis, one of the kindest and most loyal people in the entire Marvel Universe would never do. Even the later reveal that Jarvis had been replaced by a Skrull doesn't justify this, since the Skrulls are supposed to be imitating whoever they replaced exactly.
Wong suddenly turns into an aggressive, territorial man who threatens to stab Jarvis if he steps foot in "his" kitchen.
Hawkeye, a man who divorced from his wife in an argument of whether superheroes killed or not, suddenly becomes willing to try and kill Norman Osborn. Bendis has admitted to hating Hawkeye, and this would not be the last time he had Hawkeye ignore his no killing rule, only this time, he outright does kill someone. This is a case of the derailment being 100% intentional.
Wonder Man, after Dark Reign, starts ranting on about how the Avengers are a bad idea, even attacking the Avengers on several occasions without any real reason. He has supported the Avengers before and relishes opportunities to be on the team.
Viper, an infamous Psycho for Hire, constantly and consistently ranting against capitalism. She's a nihilist with a fondness for pointless killing with motivation so random even the goddamn Red Skull cut off ties with her, but Bendis seems to think she hates capitalism specifically when this has never been a thing.
The Venom symbiote being abruptly purged of bloodlust in Guardians of the Galaxy is a odd case as it throws both thirty years worth of character development and character derailment out the window. On one hand, it largely resets the Venom symbiote to how it was before the early 2000s where it was derailed into becoming a straight up villain and lost most of it's former noble traits. On the other it lost everything that made it villainous — especially the love-hate relationship with Spider-Man that has been central to its characterization all this time — while also negating the Enemy Mine/Superpowered Evil Side dynamic it had with Flash, kiboshing any reason for Venom and Toxin to have a throw down and putting a end to the side plot point of Flash slowly making it more noble again. Bendis' reason for this? To pander to new fans who don't know Venom was ever a monstrous, cannibalistic supervillain and only think of him as a "good guy with a super cool costume that can do amazing things". Ironically this seemed to have the opposite effect, with older Venom fans generally not minding or tolerating the retcon as, outside of it being forced and prematurely ending some plot lines, it largely played nice with the continuity and many preferred the symbiote's older characterization. However, newer fans actually tend to prefer how the symbiote was before due to adaptions largely making Venom as a whole into a straight up villain instead of a Noble Demon turned anti-hero.
A common complaint of Fantastic Four fans is that almost every time a new writer takes over, they ignore years of continuity and regress Johnny Storm back to being an impulsive teenager just so they can do the same "Human Torch learns responsibility" storylines done before.
In Mark Millar's run on the book, Johnny is shown as a guy who changes careers on a whim, can't care about anything but his fame, and sleeping with a hot thief right after she's pulled a robbery.
On the X-Men page linked above, it's been noted that Warren Ellis's approach when writing for mainstream comics lately has been "take a look at past few issues featuring character, extrapolate from there as baseline behavior." If you want any further proof that's the case, then behold as Captain Americacondones torture.
While his ideals and so on were kept intact, Cap's being forced to marry the Idiot Ball probably counts as derailment. The writers forced him to adopt a strategy that had literally zero percent chance of achieving any lasting peace or useful result, because otherwise the storyline wouldn't have had the ending they wanted. And then along comes Avengers vs. X-Men which is such a blatant betrayal of everything Cap stands for and believes in that it's quite frankly offensive.
Iron Man. Just a special mention for getting the worst. Going from a hero (albeit occasional jerk) to Der Eisenfuhrer, the armored superhero who doesn't mind throwing his best friends in jail for life. It took over a year and the movie to undo much of the derailment and there is still a very vocal Hatedom. Unfortunately, most of the people who decided to check out the comics because of the movie quit reading out of disgust, so the Hatedom is still the primary chunk of readers.
Quite a few of those long-time fans will argue that the real Iron Man has been dead for quite some time; the "Tony Stark" in the Marvel Universe right now is just an amalgam between the lame-ass teenage Tony Stark and the Heroes Reborn Tony Stark. However, officially, the Tony Stark who died was indeed resurrected and merged with teen Tony as an Avengers annual reveals why certain characters were changed back to their pre-The Crossing incarnations—and Tony is one of them, having unmasked himself to see the classic Tony Stark looking back in a puddle, having the memories of his own life—including his death, and the memories of the heroes' time in the Heroes Reborn and teen Tony's life—and his own grave being empty with an energy signature coming from it and traces of it in his own body.
Due to Executive Meddling, pretty much everyone on the pro-reg side got this treatment retroactively as well, because when it was started, the pro- vs. anti- registration debate was supposed to be nuanced, with both sides having good and bad points, and therefore supporting pro-reg side wasn't that out of character for Stark et al. However, the pro-reg side soon was Flanderized by the writers to near-Nazi levels of evil, while its original supporters apparently became complete morons and didn't change their opinions when they realized things were going to crap. Stark got the worst of it, but others, including Hank Pym (though he was later revealed to have been replaced by a Skrull during the event's time frame, so his characterization retroactively managed to stay intact) and Reed Richards, had their characters pretty badly shafted too, especially as the latter was responsible for once giving Congress one of the best anti-registration speeches ever delivered. By the end, the plot had seemingly devolved into rival factions of writers demonizing whatever side they disagreed with to a ridiculous degree.
Except that even then, it would still be completely OOC for Tony. Most of the mainstream dismay over Tony's derailment in Civil War seems to stem from the fact that Tony is a good guy, and while he might not be immune to ever making minor morally ambiguous actions, he would never under any circumstances attack his lifelong friends and take away people's rights. But even if Tony wasn't a frickin' hero and good, moral person; it would still be OOC because Tony is usually portrayed as a libertarian who hates government control and oversight of superheroes, is extremely suspicious of and unimpressed by proposals to regulate and record sensitive information about his armor technology, tends to be paranoid of people he doesn't know and trust on a personal level due to how many times he, his friends, and his team have been endangered or compromised due to meddling government officials or information falling into the wrong hands, and has sacrificed most of his influence and reputation, a good chunk of his fortune, and on more than one occasion, his mental stability, on keeping The Avengers safe from society and the government by managing, organizing, funding, and legitimizing the team and acting as liaison between it and the federal government, the police, the New York City courts, and various other organizations. If he absolutely had to turn into an immoral supervillain-esque monster, he'd go off the exact opposite side of the deep end into some sort of anarchist rebel.
Tony at least had some means of absolution that vaguely fit the character. Minor characters like Sally Floyd and Ben Urich effectively had their sympathy nuked off the face of the earth - first by telling Captain America that he (representing the ideals of America) was outmoded because he didn't care about the same pop culture as the rest of America (an argument that might have had some traction if Cap hadn't sat there and taken it), and then - keep in mind, these characters are journalists - handed over information to Tony about how one of his underlings tried to trigger a false war with Atlantis to gin up sympathy for the pro-reg movement because "the ends justify the means." The situation with Sally Floyd is especially startling compared to her debut in Generation M, where she had valid criticisms against a xenophobic senator's arguments for mutant registration even after most of the mutant population was depowered. To see her go from being a staunch supporter of mutant rights to making such bullshit comments about popular culture only goes to show how badly the Executive Meddling in Civil War ruined a lot of characters.
Civil War II has now done this to Captain Marvel. She went straight to imprisoning people for crimes that they may or not commit and caused some prophecies that she wanted to prevent to come true, all on the word of a new Inhuman precog. Even worse, she still kept going when it was revealed that he wasn't really seeing the future. In the end, she is seen as a hero even though she pissed off most of her friends and almost killed Iron Man (he's in a coma thanks to his own experiments).
Ed Brubaker did this to Black Tarantula in his Daredevil run. Black Tarantula was created as an elegant boss of a criminal empire who put himself above everybody else, is an heir to ancient power and Implacable Man who goes toe to toe with Humongous Mecha. Brubaker completely changed his character, gave him father issues, turned his empire into an ordinary gang and weakened him without giving any reason. Some say that he's gone from a badass sinister crime boss to a low-level stereotypical thug, while others point out that in certain Spider-Man stories he was getting really close to Invincible Villain territory (he defeated Spidey twice and spared his life because he didn't see him as a real threat).
Canonically, Death's Head II is supposed to be the same character as Death's Head. However, many fans felt he was not the same character — he was not written by the original writer, exhibited none of the mannerisms and personality quirks of the original, and ended up as a generic Darker and EdgierInvincible Hero. It got so bad that Death's Head's original creators Simon Furman and Geoff Senior wrote and drew What If?... #54 just to show their take on what should've happened instead.
Ares, master strategist and badass god of war turned into a stupid brute who only says aye.
Weapon Omega went from a guy who was always afraid of using his powers and never wanted to hurt anybody to a psycho who likes killing people.
In the new Defenders series, we now have Doctor Strange, sexual predator, and Betty Ross, slightly petulant adrenaline junkie who keeps forgetting she's invulnerable.
Fraction also has a pretty obvious personal dislike for Doctor Doom. During FF, he pulled no punches on this dislike, having him beaten by Scott Lang, who also reveals him as just an amateur magician and a self-mutilator, calls every redeeming quality Doom ever had a straight-up lie (and compares him to several real-life dictators in the process).
Odin, under most writers, is a wise and benevolent if short-tempered pompous leader with a decent respect for humanity and is three steps ahead of his enemies. Under Matt Fraction, he was a genocidal war monger whose only solution to a problem he knew was coming was "kill them all", and this is also his characterisation under Jason Aaron, who also adds misogyny to Odin so that he can have Odin embody "The Patriarchy".
Attempts to break away from their own title for the stars of Power Pack have led to baffling character derailments, such as when eldest boy Alex stole his brother and sisters' powers and became the laughably-monikered Power Pax, alienating fans for years after the event. Eldest girl Julie Power also changed from a book-reading, highly-articulate redhead (her character was based on her creator, Louise Simonson) in the original comics to a bumbling, dumb blonde actress in the pages of Runaways and a later spin-off series, The Loners, where she was notable for magically appearing in one scene without explanation merely to be brutally stabbed so that a male character could be shown to feel guilt at her situation, and having a solo story that somehow convinced several thousand readers to stop buying the book with only two issues to go. Still, the "dumb blonde" part was later shown to be a deliberate facade.
The Punisher has had to brush with character derailment ever since he was created. Although his exact quirks and personality vary slightly from writer to writer, in his heyday in the early nineties he was generally portrayed as sincerely wanting to help people and keep them from going through the same things he did, would occasionally question his actions and show mercy if the situation warranted it. Even a few traditional heroes considered him a good man at heart. Other writers would instead portray him as a complete amoral psychopath who didn't care about anything except killing criminals, and never ever questioned his actions.
Garth Ennis struck a fine balance with The Punisher MAX, portraying him both as a complete amoral psychopath who doesn't care about anything except killing criminals and who never ever questioned his actions, but also as a man who is extremely diligent, going to huge lengths to ensure absolutely no innocents are harmed while he's "working". And on rare occasions, the plight of those around him do get through - just read "The Slavers".
Quite possibly the ultimate derailment was the miniseries' Purgatory and Revelation, which set Frank up as a vigilante in service for heaven. In fact, these were the last Punisher stories until The Punisher: Welcome Back, Frank, wherein Garth Ennis promptly undid that idiocy.
The last arc of Runaways featured every character getting derailed to some extent. Victor Mancha accidentally kills Old Lace through a stupid stunt, Klara goes berserk and keeps screaming at everyone, Chase threatens to break every bone in Klara's body, Karolina nearly makes out with Chase, Nico actually does make out with Chase... And then there's Gert, who's suddenly alive again, and instead of being snarky, she's almost cheerful and kind of flirty...
Most damningly he makes the deal in One More Day to evade the consequences of his own actions, despite taking responsibility no matter what it costs him being the very core of his character. All to save an aging Aunt May who he knew accepted their death and absolutely would not have wanted Peter to give up his marriage for them.
For that matter, Mary Jane Watson as well. In One Moment in Time, said deal is retconned so that she is now the one to have agreed, and adding insult to injury, she says, "See, to me, the reason I wanted to get married was to have kids. If that's (children) no longer a part of the equation, then marriage is just a piece of paper."
Then in Superior Spider-Man, Peter's Moment of Weakness that allows Otto to fully erase the remnants of Peter Parker from his mind has been resoundingly criticized by fans of the character as something he would never do, namely allowing an innocent little girl to die to keep Ock from finding out about him. As it turns out, he hadn't been erased at all.
Black Cat was subjected to this after the events of Superior Spider Man, in which Felicia was attacked by Spider-Ock and swore she would get revenge. After Peter's return she became an outright Ax-Crazy psycho who wants to destroy everything about him and doesn't care that he was under someone else's control, attempting to harm anyone and everything else in the process and even joining forces with several past Spider-Man villains to do so, even several that she used to be enemies with. She also now wants to be the toughest, most feared crime boss in all of New York despite that never being her motivation in the past. Fans of Felicia in her Anti-Hero days have not been happy. The not so subtle implication that this was all done primarily to further solidify Silkas Peter's love interest didn't help. Her later appearances in Silk and The Defenders, fortunately, depict her as a more sane and morally nuanced character rather than a flat out Psycho Mob boss.
And thankfully, recent events have led to Cat openly called on how she just doesn't have what it takes to be a truly ruthless mobster and thus goes back to her cat burglar ways. A later talk with Spider-Man reveals that Black Cat subconsciously knew that they had a closer relationship prior to One More Day mindwiping everyone of Spidey's identity, and she was deeply hurt that he never talked to her about it despite their relationship now being drastically altered. Peter re-revealing his identity to her cemented that she's solidly an Anti-Hero once more.
J. Jonah Jameson has always been a jackass (whether there was anything morethan that depended on the writer) but he also tried as hard as possible to reveal the truth about any Villain with Good Publicity. Later stories have his hate for Spider-Man go from irrational to certifiably insane and him becoming a vocal supporter of Norman Osborn, a man who treatened to kill his family in the past. While JJJ's given many reasons in the past for his hatred of Spider-Man, generally it has something to do with a deep resentment of his casual disregard for the law and the way his vigilantism breaks said law. Come Superior Spider-Man, when a Doc Ock-possessed Spider-Man starts brutally beating criminals and being far more vicious, killing one in cold blood, he roots for the 'improved' Spidey, even at one point asking him to kill a supervillain for him. He loses this new-found respect for him when Slotto blackmails him into giving him the means to become Big Brother to NYC, but the point stands that now, according to Dan Slott, JJJ didn't hate Spider-Man because he broke the law, but rather because he didn't break the law enough! Not to mention, there's all those police officers who help cover up the casual murders because they now respect Spidey more for them, despite previous storylines establishing that, with small exceptions, a lot of cops dislike Spidey to the point some have tried to frame him for murder.
Spider-Man Beyond sees Ben Reilly continue to be treated with a great deal of contempt by Marvel following his deeply divisive portrayal in Clone Conspiracy. He is portrayed first as a naive corporate stooge, at odds with his classical characterization as more cynical and distrusting of others, before being mentally altered by Beyond into becoming a selfish jerk whose heroic qualities are slowly rung out of him, culminating in his adopting a new identity as the villain Chasm. To say that fans were not pleased would be an understatement. Editor Nick Lowe’s defense of the decision only added fuel on the fire.
At the end of Venom Inc., Andi Benton abruptly decided to leave her symbiote with Lee Price despite having spent the entirety of the event trying to rescue it from him; and made no attempt to remove it from him after he was defeated. Even her creator, Cullen Bunn, was shocked by this development.
In Empyre, the Cotati, originally depicted as a peaceful race and allies to the Avengers, are suddenly secretly evil all along and have been planning to murder all animal life in the galaxy since the Kree genocided them as a result of the Skrulls setting up a "Who wants to be uplifted?" Contest. Their motivations for this are not only poorly thought out but incredibly hypocritical and they do various heinous, horrible, monstrous things in order to justify the need to put them down in as brutal a fashion as possible. And the Avengers trusting them implicitly and wanting to defend them from, what appears to be, the Kree and Skrulls coming to finish the job, is presented as naive at best, stupid at worst. Iron Man feels guilt specifically for essentially trusting the Cotati and their whole "victim" story. To repeat, he is upset over how he trusted the victims of a genocide and not the people who committed it. Worse yet, any discussion about what the Kree did to the Cotati is largely glossed over in favor of painting them as wholly unjustifiable in all their actions. The Cotati in "Empyre" are basically just horrible monsters who apparently deserved to get slaughtered wholesale and are only spared that fate because they basically surrender and, from all appearances, become slaves to the new Kree/Skrull Empire. Tone-Deaf much, Marvel?
Longtime Captain America villain and former leader of the Masters of Evil and Thunderbolts, Baron Helmut Zemo, suffered this sometime around 2010, which was extra tragic as the character had undergone seriousCharacter Development across the last decade, and complicating matters, it seems this was done for the sake of deliberately undoing the aforementioned development in an attempt at Character Rerailment. Over the course of Thunderbolts, Zemo developed from a selfish Neo-Nazi Manipulative Bastard to a genuinely well-intentioned Magnificent Bastard, one whose only goal was to save the world, whose arc culminated in him confronting his abusive father and angrily rejecting the tenants of Nazism and fascism in general. He seemed to have firmly became an Anti-Hero, but then he suddenly dove back into villainy by antagonising Bucky Barnes and Clint Barton out of petty jealousy (the latter due to previously-unmentioned hatred he had for him over him causing Karla Sofen's breakdown, and the former due to jealousy over Bucky's perceived Easily Forgiven nature), but at least demonstrated he was still a tactical genius. Over time though, he became more villainous and once again a fascist (yet was also running a country that operated as an anarchic state), and became an utter joke in a fight. He got involved in crimes he'd never have indulged in even when he was a genuine villain (such as child trafficking), while seemingly falling back on his reputation rather than any actual skill or merit, utterly misunderstanding the motivation he developed.
Thanos: From the seventies to the early nineties, Thanos was mostly a consistent character with most of his stories written by Jim Starlin. Thanos started out as a cataclysmic villain who saw himself as a hardcore nihilist. He tried to convince Death herself to love him, while failing to understand what she really wants. He developed into a more mature antihero that has achieved self-clarity with the help of Adam Warlock. Then after the Avengers (2012) post credits scene featured him, Marvel decided to undo all of his character development and make him a thuggish and simplified space villain like Mongul or Despero.
The veteran Avengers foe Master Pandemonium has suffered quite a bit of derailment in recent years. He is a powerful magic user, he controls demons, he served as a major foe for the West Coast Avengers, and he has served as a Stalker with a Crush for his archenemy, the Scarlet Witch. In the 21st century he has been demoted to a henchman for various villains, he has worked as a school teacher and cook, and he has been depicted as a mentally ill housemate for the hero Speed.
The nocturnal hero and vigilante Shroud has had his own share of derailment since the Civil War (2006) event. He has always been obsessively devoted to crime-fighting and quite willing to use people as pawns. In recent years, he turned into Madame Web/Julia Carpenter's Psycho Ex Boyfriend, a super-villain in his own right, and a cold-blooded killer.
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Character Derailment technically never happened as far as his memories are concerned, and Steve doesn't want to talk about it.