This is often someone who starts out with a specific goal, as a Well-Intentioned Extremist or someone out for revenge against the perpetrators of a particular wrong (real or imagined), or someone who is driven to actions by unfortunate circumstances. This can make a very deep and often tragic character, but it's hard for the writers to keep it up, especially in a long-running franchise. Perhaps the author simply has trouble following through on their original idea, other parts of the story got more important, or the creator was making it up as they went along. Perhaps later writers who come on in the middle of a work's run just don't "get" it. Sometimes the writer is trying to give some kind of Aesop that because their original motive was grey, this leads to monochrome quickly. Too quickly. On the other hand, it sometimes is intentional. The character may very well have lost track of what they were fighting for and kept going on in spite of this because they have nothing else left.
This often happens when a story has several antagonists, one of whom is only needed for conflict. Expect the character to become a cartoonishly evil megalomaniac. If this happens to a side character, it is often followed by Flanderization. Perhaps even more strangely, the other characters won't notice such decay and will, at the least, be unsurprised that this character is now a walking cliche. If this happens to a series protagonist, expect a Dork Age, gnashing of teeth and shrieks of Ruined Forever!
This is, if anything, even more common in stories told out of order, as a writer wishing to give a two-dimensional character more depth will go back and write a backstory exploring the reasons behind his good or bad actions, especially as the character tends to be the protagonist of the backstory.
Comics do this a to a certain degree, as various writers are more or less interested in character depth than another writer. This is common in serial works in general for another reason — usually, a villain becomes popular for what they do as a villain and not for the reason they do it, so when they want to keep using a character, writers have an incentive to discard motivations that might get in the way and put them through Flanderization down to their most essential parts. A particularly common form of this for B-list villains is for them to have a clear and understandable motive the first few times they appear, which eventually decays into "get revenge on the hero for the last time they defeated me" ad infinitum.
As villain motivation isn't always seen as integral to the plot, expect villain examples. Compare The Dark Side Will Make You Forget, where the changing of motives is a form of Character Development. Similarly, It Gets Easier, and He Who Fights Monsters are in-story development of motives. Likewise, this may be purposely invoked in order to create a Self-Disposing Villain.
- Originally in Lily's Changes, Tom Riddle created a Horcux after accidentally killing his father because he was certain it was the only way to protect himself should Dumbledore learn what he did. Splitting his soul drove Tom insane and caused him to start creating more horcruxes and become a dark lord.
- In the epilogue of the Princess of the Blacks series, the Longbottom family and half the Granger family a part of a feud with the Black family nearly a century and a half long, but only Jen Black is old enough to remember why and she's mere minutes from dying of old age.
- In Nobody's Hero, Ai reveals he wanted to do a Suicide by Cop because there is a self-preservation protocol built in him, necessitating for him to escalate things until he's killed. But two years of grief blinded him and he forgot his original intention, rather, his desire warped into terminating humanity.
- The Swan Princess: The villainous Rothbart's initial goal is to take over King William's kingdom. He kills William, then kidnaps his daughter Odette, turns her into a swan, and will only set her free if she agrees to marry him, making him legally the next king. So far so good. But when he learns that her true love Prince Derek (the only one besides himself who can break the spell, with a vow of everlasting love) has found her, he resolves to kill them both. It never seems to cross his mind that killing Odette will ruin his plan to legally claim her father's throne.
- Kariya Matou's motivation for entering the Holy Grail War in Fate/Zero is that by winning, he'll be able to return his adoptive niece Sakura to her mother, a woman he loves from the abuse of the Matou family patriarch Zouken. However, the strain of maintaining a Servant like Berserker and the Matou crest worms eating away at him causes his mind to deteriorate and he becomes obsessed with killing Tokiomi Tohsakah (Sakura's father, the man who unthinkingly gave Sakura to the Maotus).
- Murtagh from Inheritance Cycle was derailed from a sympathetic villain who works for the Big Bad in order to make the world a better place to a one-dimensional snickering cliche that rivals Snidely Whiplash. This could be considered a Take That! to how he was becoming something of a Draco in Leather Pants to certain sections of the Hatedom.
- It doesn't help that Galbatorix made it impossible for Murtagh to convert to Eragon's side. Really, someone who physically can't choose for himself which side he fights for is going to be miles more sympathetic than someone who can choose and mindlessly calls a person evil when that person, again, has no choice in the matter.
- Victor Helios in Dean Koontz's Frankenstein series first desires to eliminate humanity and replace it with a masterrace of his own creation, and eventually take over the world and the universe. In the sequel series, his clone who has his memories and personality now just wants to wipe out humanity and then kill himself when it is completed for some reason. This is explained by Koontz as the next logical step to the original motivation, even though they are two completely different things.
- Artemis in the book Acheron, one of the most recent books in Sherrilyn Kenyon's Dark Hunter series starts out as a friendly and curious goddess at the beginning of the book. At the end, you just want to smack her for her unadulterated meanie abilities.
- Older Than Steam: Satan in John Milton's Paradise Lost initially views corrupting mankind as a continuation of the fight for freedom from God. When he realizes that his rebellion against God has been a terrible mistake from the start, he decides that he can no longer be good and must embrace evil as his good. Later on, his motives further degrade to something akin to "Since I'm evil now, I must cause God as much trouble as possible." This decay is likely deliberate, as the whole arc of Satan being diminished in mind (as well as stature — from towering angel to serpent) exemplifies just how destructive evil can be to its practitioners.
- Related to the Satan example is Sauron in Tolkien's legendarium — he starts out as a clever, Manipulative Bastard in The Silmarillion, who works with Morgoth only as part of his agenda to bring order to the world, then gradually degrades to the Kill 'Em All type seen in The Lord of the Rings. This was a deliberate piece of decay on Tolkien's part, due to his belief that evil usually starts out with some kind of high-minded ideal but its methods (and Sauron's multiple deaths) eventually ruin its practitioners, leaving them arrogant, hate-filled, mindless destroyers.
- Even more pronounced is what happened to Sauron's former boss, Morgoth (who is intended to be Satan). He started out wanting to create and be the God of his own universe. But as he realized that nobody but God could actually be omnipotent, he gradually descended into a pure Omnicidal Maniac, with a corresponding loss of his power as the mightiest angel. At least Sauron kept the same basic goal (wanting to rule the world) in mind the whole time, even if the reason shifted from "I'm the only one who can run things right," to "UNLIMITED POWER!".
- Played straight as an arrow in the Star Wars Legends Legacy of the Force book series. In the first book of the series, Jacen Solo has visions of galaxy-wide destruction, including his dueling and killing Luke, that can only be prevented by accepting the teachings of the Sith. Lumiya, a former Vader apprentice, softens the blow by teaching Jacen that through careful discipline and sacrifice, he can avoid falling into darkness like Vader and the Emperor. However, with each successive book, his altruistic motives are shown less and less, slowly replaced by ever-increasing anger, distrust, intolerance, and desire for power. All of the culminates when he kills Mara Jade, who had caught onto his downfall, for a reason (or at least a rationalization) that makes almost zero sense and is later completely abandoned by the series. In a later book, he Force-chokes a young lieutenant to death for a perceived error, despite the fact she followed all standard procedures and was commended earlier in the book for her attention to detail. At this point, almost all trace of his benevolent motives are gone and all that remains is the Dark Side.
- As others have noted, Motive Decay seems to be an occupational hazard of Sith Lords. Every Sith whose motives we've had an opportunity to examine started down the "dark path" for reasons that would make sense to normal people. Some want to unite the galaxy (decent motive), or to protect loved ones (ditto). And yet every Sith we've seen has degenerated into committing acts of sheer evil. Even those who from the start had selfish motives of personal power tend to decay into cruelty for its own sake. This suggests that the trope in question is intrinsic to the Star Wars concept of The Dark Side. In the Star Wars galaxy, Sith's motives decay themselves.
- We see this happen to Ysanne Isard, head of Imperial Intelligence, over the course of the X-Wing Series. In the first book, Rogue Squadron is just another enemy asset to be destroyed, while Isard's attention is still on her wider Xanatos Gambit to destroy the New Republic. By book four the Rogues have thrown a wrench in her plan, one of them "defiled" her secret prison by escaping it, and the squadron has gone on to wage a guerrilla war against Isard's power base - so she lets it get personal and starts prioritizing hurting them even when it goes against the rest of her interests. Even Isard recognizes her Villainous Breakdown, but by that point she's too far gone to change tactics, while her subordinates begin to call her out and desert her.
- Emperor Jagang in the Sword of Truth books starts out as a man who believes in human superiority and unlimited potential, and believes that magic prevents the men from creating a technological society. Later, he is a religious fanatic/Omnicidal Maniac who believes that all men are evil and unworthy of life, and it's his defeat which causes a sudden technological development. At the point of his change, the books take a strong right turn into Objectivist didacticism, which suggests a different trope altogether is in play.
- Reversed in the Vorkosigan Saga, where the Cetagandans were introduced as pretty generic bad guys, defined by militarism and expansionism, but were soon developed as a society with hints of a history, changing goals, and internal disagreements.
- In the Heralds of Valdemar series, this explicitly happens to Big Bad Ma'ar, alongside and combined with his Villain Decay. In the Mage Wars he was a Well-Intentioned Extremist who united the barbarian tribes and sought to impose a Utopia Justifies the Means. Through successive reincarnations and overuse of Blood Magic, he eventually devolves into a twisted, megalomaniacal schemer whose grandiose plans to Take Over the World end up thwarted by generation after generation of heroes. This all turns out to have been a scheme of the gods, who needed his knowledge to avert the return of the magical Cataclysm that started with his original death.
- Proof once again that Tropes Are Not Bad, Motive Decay is both Played for Drama and Played for Horror in the ending of Worm. The first of the two most important is Big Bad Scion, who has mostly given up on the cycle of his species and entirely given up on trying to find an emotional payoff in selflessly helping others. Instead, he decides to find that emotional payoff in the simplest way available to him: killing, maiming, and later torturing on a multiversal scale to revel in the chaos buried in the human psyche. The second is Khepri, and we get to watch from inside her head as she slowly loses everything that made her heroic, all of her identity, and finally her humanity after she unfettered her shard in a desperate gamble to gain an advantage against Scion. Her Goal in Life goes from saving humanity throughout the multiverse to forcing everybody to cooperate, and finally at the end of it all, when she has lost the ability to comprehend the world except through aggression, to a desire to systematically kill every other being in the multiverse to eliminate their threat to her so she can have peace.
- Intentionally explored in The Machineries of Empire with Nirai Kujen. He started out as a Well-Intentioned Extremist who genuinely wanted to create a society of peace and prosperity even if it was a totalitarian state with no free will and perpetual war with all its neighbours, but when he found himself tormented by conscience he intentionally turned himself into a sociopath via brain surgery and psychological conditioning. Unfortunately, he didn't realise that he would completely lose his ideals and end up being indirectly or directly responsible for centuries of torture, repression, imperialism and genocide, solely so that he could live forever in luxury.
- In The Sneetches and Other Stories, the Star-bellied Sneetches initially use their stars to flaunt their superiority over the plain bellied ones. When the latter actually obtain stars, however, the original star-bellies protest, believing they are still the better Sneetches regardless, and remove their stars and consider plain bellies to now be the top option. The process repeats until none of the Sneetches on either side can even remember what clique they were in originally.
- After Dixie and Izzy of Special K lost the ROH Tag Team Title belts to the Briscoes and then the whole group was beaten by Generation Next, they became depressed and eventually turned on one another. Lacey's Angels was initially about regaining their glory and tag title belts but Lacey eventually started caring more about her own reputation, which lead to firing Izzy and Dixie for BJ Whitmer and Jimmy Jacobs. This lead to her insisting Jacobs change in various ways to the detriment of her tag team's performance, which in turn lead to Whitmer leaving in disgust as Jacobs bowed to her every whim. Eventually tag team ambitions fell to the wayside completely as Lacey became more focused on her vengeance against Whitmer and then Colt Cabana for refusing to put up with her increasingly self centered practices.
- When Ric Flair betrayed his stable Fortune to side Hulk Hogan and Eric Bischoff's Immortal alliance in 2011, it pretty much did away with all his sense of direction in TNA, both in- and out-of-universe. In-universe he betrayed his own plans to both finally defeat Hogan once and for all and to remake The Four Horsemen with TNA homegrown talent and proved it was all about the power for him. Out-of-universe he seemed to float around with no purpose, as there wasn't exactly much potential to elevate amongst the Immortal ranks, what with the one young rising star in Gunner being lost in the shuffle at the time. He was such an ineffectual member with Immortal that people were speculating he was being The Mole to help either Fortune or Sting with taking out Hogan, but this never materialized.
- Speaking of Immortal. Jeff Jarrett's motivation for joining was to gain control of his company back from Dixie Carter. However, there was no resolution to this as he would soon go on and feud with Kurt Angle.
- In 2013 The Decade started out attacking Eddie Edwards, who was leaving Ring of Honor for TNA, saying they were tired of seeing people celebrating when wrestlers leave for larger companies. Then they switched gears to attacking wrestlers returning from larger companies like The Addiction. Then they started attacking wrestlers who worked for ROH part time while contracted by larger companies(reigning IWGP Champion AJ Styles). Then they started attacking new wrestlers they suspected of trying to use ROH to get noticed by larger companies. Their focus had shifted a great deal, but at least there was a consistent theme. But in 2015 BJ Whitmer tried to turn Decade into his personal army against Steve Corino, which ended up destroying the group.
- Doink the Clown's original gimmick was that he was a talented wrestler - not only was this back when then-WWF acknowledged their product as wrestling, but Doink's wrestling skill was repeatedly emphasized by the commentators - who was dressing up as a clown to play mind games and prove a bizarre point about the wrestling business that only he actually understood. The mind games turned into pranks, the pranking gradually took precedence over the 'talented wrestler' part of the character until it was forgotten about, and he ended up turning face and finding a midget buddy.
- When the heel stable RETRIBUTION debuted in WWE in late 2020, they acknowledged their goal was to upend the current WWE system, which they felt had used and abandoned them, and rebuild it. So after they were given contracts they set out to accomplish this goal by...targeting random mid-carders. Not the WWE champion or anyone with titles, just mid-carders with no real importance, who are essentially in the same boat as their reason for forming in the first place. This is one (of several reasons) why the group has become the laughingstock of the wrestling world.
- The godmodder in Destroy the Godmodder suffered from motive decay extremely quickly. The godmodder wishes for everyone to rage quit for eternity, but that very quickly got/gets lost sight of in the seas of massive explosions and energy blasts.
- Warhammer 40,000's Chaos Gods, like the Dark Side, are quite fond of corrupting the motives of the fools who sign up with them willingly. The Horus Heresy novels show how this happened to most of the traitor Primarchs:
- Horus' descent into evil is initially triggered by the Emperor's near suicidal mysteriousness about what his intentions are once the Galaxy is united under his rule. Horus, not knowing what the Emperor plans to do and aware of how the Space Marines are being increasingly sidelined, rejects this and accepts the idea that as superior beings, the Space Marines have a right to rule. When the Chaos Gods show him a vision of a grim, dark future in which the Emperor is worshiped as a god and Horus and his brothers have been obliterated from history, Horus decides to overthrow his father for the good of mankind. But by the Siege of Terra, hundreds of billions of deaths later, Horus seems to have accepted the Chaos Gods and is mostly in it for horrendously evil gits and shiggles. In short, he went from "I will save the Galaxy" to "I will rule the Galaxy" to "I will destroy the Galaxy".
- Konrad Curze, the Night Haunter, started off as a Well-Intentioned Extremist who believed in maintaining order through fear, tenets he passed on to his Legion. Curze went mad after the Heresy and let himself be assassinated to prove that the Imperium was Not So Different from him. In the years since, his Legion has mostly forgotten his ideals and spreads terror For the Evulz.
- Magnus the Red was devoted to advancing humanity through the acquisition of knowledge, but had an unfortunate taste for forbidden knowledge. Magnus was forced to join with Horus to survive the Emperor's wrath, and since then he's been obsessed with getting revenge on the Space Wolves for burning his planet.
- Fulgrim started off pursuing perfection, but even before the Heresy he'd become an arrogant Jerkass who believed he represented perfection. Chaos played on his arrogance and twisted him into a hedonist. He's only gotten worse since then.
- Alpharius and Omegon, as revealed in the book Legion, sided with Horus to ensure that Chaos ultimately lost the war. There are several problems, however: first is that the Alpha Legion is notoriously decentralized and secretive, second is that Alpharius was reportedly killed during the Heresy, and third is that Omegon at some point began plotting behind his twin's back. The net result is that we can't be sure who the modern Alpha Legion is following or what their ultimate goals are, assuming it even has a unified agenda anymore.
- A lot of Chaos cults start out as organizations for actual reform (removing corrupt leaders, purging mutants, killing criminals...). Unfortunately, while a lot of what the Imperium does is stupid and wasteful, a lot of what it does is also Necessarily Evil, and Chaos is all too happy to exploit this.
- Heroes in Beast: The Primordial originated as a sort of balancing tool for the Primordial Dream: their job was to keep Beasts in check. Modern ones have largely, although not universally, degenerated into callous narcissists interested solely in killing Beasts for glory.
- Delta Green: What leads to Majestic-12's demise. The group originnaly made a pact with The Greys to have their technology in exchange for allowing the greys to experiment on humans, as the MJ-12s Steering Committee saw as the only way humanity could actually compete and fight with the other more powerful aliens. Some elements inside it started to see that MJ-12 degenerated into almost woshipped Grey technology for its own sake like a Cargo Cult instead of protecting the United States and Humanity as it was their original mission. Eventually those who thought like that work to take down Majestic from the inside with the help of Delta Green and turn into "The Program".
- Adam Taurus in RWBY is a subversion of this trope. Thanks to Blake's initial abuse-influenced description of him "changing over time", many people both in and out of universe interpreted him as a Faunus rights activist gone too far. However, his volume six character short demonstrates that he was always emotionally manipulative and never hesitated to use lethal violence, only briefly showing concern when his superiors reprimanded him. That said, his lust for power and refusal to accept consequences did start out as fairly hidden, only becoming obvious over the course of the show.
- Robotnik in the titled "Robotnik Finally Wins"  robotizes Sonic and Tails and then all the animals of the world... then spends the whole video lampshading this trope trying to remember why he was spending decades even trying to do that in the first place. Turns out, he was trying to make members for a bagpipe band.
- In The Adventures of Dr. McNinja, throughout most of the story, the Big Bad, King Radical, has the main goal of saving his world and people by sacrificing the Doctor's world. But when the Doctor ruins his plans beyond repair, the only motive King Radical has left is to make the Doctor suffer as much as possible.
- Thief from 8-Bit Theater stole only to pay for his father's medicine. After the whole issue was resolved, he kept being as greedy as ever. When pressed by Black Mage, he was asked "So shouldn't you, y'know, stop stealing?", to which the answer was a simple "No." Every single Elf is a total bastard. It's even part of their (stolen) national anthem.
- An example in this comic from The Adventures of Wiglaf and Mordred, except the evil one is the one who was attacked, not the attacker. Gawain, the attacker, genuinely forgot why he was attacking Arthur in the first place because he is a zombie with a Healing Factor, and while shooting him in the head won't kill him, he loses and regains random pieces of memory every time. Even though he couldn't remember his original reason, he figured that revenge for being shot in the head so many times was still a decent reason.
- Muh Phoenix: The comic doesn't miss the opportunity to point out that, after getting the power of the phoenix, Cyclops never tried to restore mutantkind (he decided to form a rap band with the other Phoenix hosts).
- Yet Another Fantasy Gamer Comic: In-Universe example, Lewie the Lich King's original goal was to get sweet, sweet revenge on the barbarian pirates who sacked his kingdom, but The Fog of Ages and his manipulative goddess convinced him he wanted to rule the world. Once he succeeds in the latter, his oldest living rival asks him if he finally got what he wanted, giving him enough clarity to realize he'd been duped into becoming a Puppet King and pull a HeelFace Turn.
- El Goonish Shive: In the Legend of Diane storyline, we see the eponymous girl reflecting on how her serial dating got started, and how she gradually went from "maybe this one will be the one" to "we're all just kids hanging out and having fun" to "boys are for buying me things". Ironically, this reflection actually highlights her Character Development since her first appearance and the storyline serves as her Start of Darkness.
- Nicholas from The Care Bears Movie just wanted to cast a spell over the two remaining people in the world to get them to feel hatred. In the Dwedit Jamez Bond flash movie, he now wants to take over the world.
- Marik from Yu-Gi-Oh! The Abridged Series initially wanted to kill Yugi in order to take his screen time. Nowadays, Marik gets tons of screen time, even more than Yugi and his friends, yet he still tries to murder Yugi.
Marik: "No, I don't want to kill you. I just want to destroy you a little is all."
- Justified both in-universe and out. Marik has never been quite sure of exactly what he wants to do to Yugi, and the show ultimately has to follow the basics of the original plot, albeit barely.
- This video, which is a loose history of the Soviet Union and post-Soviet Russia (as told through the eyes of one humble worker) shows this happening to the speaker. Initially he rebels against the cruelties of the Tsar to create "a regime of peace and love" but just a few stanzas later after the defeat of the Nazis, he celebrates the thought of nuclear war with the US. The decay in his motives however is truly complete when the Soviet state falls and he enriches himself on the free market.
Now the markets are free/so much money for me/tell me why should I care for peace and love?!
- Note that all around him the stage and background are going haywire and falling apart, (corresponding to the condition of Yeltsin era Russia) and yet he doesn't care about the fact that conditions for the people are essentially the same as they were under the Tsar, except that he's now the one with the money now. The last verse (about Putin's rise) has a completely different narrator, with the original's fate never explained.