Jack: Fall for what?
Jane: For the lie we keep telling ourselves. We do the dirty stuff to get the power. It'll give us all the good things we really want. Then we get the power, we can't even remember what goddamn thing it was we wanted it for in the first place.
The character is an explicit good guy. Either in order to achieve a goal, or by being forced into doing so by someone else, they do what needs to be done. The By-the-Book Cop beats a confession out of the Serial Killer who will go free otherwise. The Paragon uses their Superpowered Evil Side to defeat an invincible foe. The White Mage breaks their oath to never use the power of light as a weapon.
Taking such a morally grey action is a technically neutral action, and in theory, it should be possible to use it to achieve the goal in question without permanently crossing the Moral Event Horizon. But The Dark Side being The Dark Side, it doesn't work like that. By its very nature, Evil Feels Good, and there are few highs like being Drunk on the Dark Side. It Gets Easier and easier to choose the dark side. The hero begins Slowly Slipping Into Evil, and eventually, they will be a completely different person from before they crossed this line.
This is a fairly common method of enacting a FaceHeel Turn or making an evil character sympathetic — but not too sympathetic — as in a sense it makes the evil character themselves a victim of evil. Sure they had a good reason, at some point. But not anymore. In a particularly bad case, the character might not even remember what their original motive was, and substitute the old goal with explicitly evil ones. The redeemable ones can usually be reminded at the last minute, or sometimes they figure it out for themselves — though they may still keep going. Often the justification for Motive Decay.
- Death Note: While Light Yagami starts out killing criminals with the idea of it bringing about justice, he starts to slip once the authorities get on his case, as he begins to kill innocent people for merely getting in his way. It gets worse after the Time Skip, where he has fully bought into the idea of being the God of his own new world, and he is beginning to seriously entertain the idea of also expanding his definition of "undesirables" to people who are committing the "crime" of being unproductive to society.
- Sasuke is a poster child for this. While he was always firstly driven by his wish to take vengeance upon his brother for murdering their family, his time in Konoha made him connect to the members of his team. The lure of revenge was too great, until he decided to break those ties. His motives have rapidly degenerated after joining with Madara. Initially after learning of the truth behind his brother's murder of his clan, Sasuke simply wanted revenge against the Village Elders who manipulated him. Now he won't settle for anything less than the complete genocide of anyone who lives in Konoha. There's some hints that Sasuke is being influenced by something (Madara claims the Uchiha bloodline is The Dark Side), probably an Author's Saving Throw given that during the summit arc he rather suddenly turns into a cardboard villain after gaining Susanoo (amongst other things, he abandoned another set of teammates that he seemed to care about earlier, who in the previous arc had saved him from being killed).
- Orochimaru was revealed to be a victim of this. The horrors of war and realization of his own mortality gradually twisted his dream of living long enough to meet his reincarnated parents into a tangible goal of discovering immortality.
- Possibly Gin Ichimaru from Bleach. It seems he originally started plotting to kill Aizen as revenge for his underlings attacking Rangiku in her youth. Now over a century later he still tried to kill Aizen, but he was willing to wound Rangiku to accomplish that. He seems to have lost track of himself a long time ago. This may be a fusion of The Dark Side Will Make You Forget and Becoming the Mask, though. Hundred years of undercover that's actually pretty fun will do that to you.
- Ralph Werec from Str.A.In.: Strategic Armored Infantry. Even as he constantly reminds himself of why he defected from the Union, he moves further and further away from his goal of vengeance and closer to being an Omnicidal Maniac, and by the end of the series, he has turned against the very people he was trying to atone to.
- In Claymore, overusing your power will transform you into a monster who forgets that eating human entrails is quite rude. The reason that a Claymore tends to overuse her power is often to protect people from monsters much like these.
- That being said, some of these monsters (called awakened beings in-universe) - particularly if they're powerful or important enough - retain much or most of their knowledge, up to even trying to maintain friendly relationships with their loved ones in some cases (though this only seems to work out when the loved ones in question become awakened beings as well). At least one has established a fatherly connection to younger characters (including a normal human, which seems like the only food awakened beings like) a very long time after his transformation. Even then, though, he continued to be an evil bastard not far from an Omnicidal Maniac including of his own kind.
- In Pretty Sammy, Misao Amano is an evil magical girl aiming to ruin the balance between good and evil. When she transforms into a form concerned with expressing the negative, repressed parts of her personality, however, she simply does things For the Evulz and lose sight of her original goal completely.
- Chirin the lamb from Ringing Bell originally seeks out the wolf who killed his mother with the intent of revenge, but finds he is far too weak. Instead, he asks to become the wolf's apprentice, with the intention of killing the wolf once he's taught him all he knows. The years pass, and Chirin grows into a monstrous ram. He forgets his mother, forgets his mission, forgets he was ever a sheep at all. Its not until they eventually return to Chirins former home at the farm where the wolf wants him to kill the sheep that Chirin finally remembers. But by then its far too late...
- Dragon Ball Super: Originally, Zamasu just wanted the gods to be more involved with mortal affairs so they would stop misusing the knowledge giving to them by the gods. At worst, he wanted the gods to stop protecting mortals since they're not worth it. After he meets and fights Goku, however, he goes off the deep end and decides that mortals are foolish and dangerous and need to be destroyed for the peace of the universe. To that end, he kills his own master, takes his Time Ring, uses the Super Dragon Balls to take Goku's body, and murders him and his family out of spite. By the time Zamasu (now Goku Black) reaches Earth, he's wiped out all the gods in the future timeline, is clearly insane, and sees himself as the one true god of The Multiverse along with Future Zamasu. Whatever nobility he may have had in his goals is long gone.
- Nura: Rise of the Yokai Clan: Abe no Seimei's Start of Darkness occurred when his mother, Hagoromo Gitsune, was mortally wounded by humans. Upon his rebirth as the Nue, his very first act is to cast his mother, who had been through numerous reincarnations and had been trying to bring him back for centuries, into Hell.
- Vinland Saga: Once Canute ascends to the throne of England he finds himself doing more and more Dirty Business to hold on to his power, in order to honour the promise he made to Askeladd to build a better world. By the time he re-unites with Thorfinn, the stresses of his office and the constant taunts of his Spirit Advisor has made him close to a carbon copy of his father.
- In The Transformers (IDW) continuity, the Decepticons started out as heroic freedom fighters rebelling against the corrupt Senate. But after they overthrew the Senate, Megatron slowly became convinced that he and only he was right, resulting in the Decepticons becoming conquerers. That coupled with Megatron's bigotry towards organic races and his emotional and mental manipulation of his soldiers, resulted in the Decepticons fighting for the same type of oppression that they were born to overthrow.
- In Old Man Logan, Magneto (the holocaust victim) works with Red Skull (the Nazi) to bring down the heroes of the Marvel universe alongside other super-villains.
- Batman Vampire: Batman goes through this in the final part of the trilogy, Crimson Mist, after succumbing to Sanity Slippage and his vampire nature. As he falls deeper and deeper into corruption, he reflects that he can no longer understand the Heroic Sacrifices of his female allies from Red Rain and Bloodstorm, Tanya and Selina Kyle, respectively, now regarding them as foolish actions rather than selfless and noble.
- Archie Comics' Sonic the Hedgehog: Dimitri, Knuckles's ancestor, once sought to restore the Floating Island to its original home in the ground and after his plan to siphon the energies of the Chaos Emeralds was rejected he still went ahead with it, believing he could fix their home. The machine overloaded and infused him with the Chaos Energies and he was reborn as the villain Enerjak. In his new role he simply sought a technological empire with him at its head, Drunk On The Darkside with his God-like powers. After being de-powered though, Dimitri's sanity gradually returns and he eventually pulls a HeelFace Turn, his old beliefs in helping his people having resurfaced.
- In Child of the Storm, as in The Dresden Files, Dark Magic has this effect, consuming all good intentions and replacing them with a desire for power.
- It's worth noting that wanded magic users have less of an issue with this due to their wands insulating them (though as is also pointed out, if you're using Dark Magic, you're likely well down this path anyway), and it's actually a requirement for the Sorcerer Supreme in this universe to be able to use Dark Magic without going bad, and many of the nastier dark wizards and witches throughout history were those who failed at this.
- This is the main reason the Dark Phoenix is such a threat as the Phoenix by nature amplifies emotions, and is amplified by them in turn: "Good becomes great, bad becomes worse." A key example is the original Dark Phoenix, Surtur. He started out as a benevolent mage-scientist on a dying world, who begged for the chance to save it and his people, summoning the Phoenix, the Enigmatic Empowering Entity in question, then pointedly breaking the summoning circle because all he wanted was to plead his case, not try and bind Her... and then ultimately spiralled into A God Am I into a desire to destroy everything and rebuild it 'better'. Oh, and in the process, he destroyed an entire galaxy. Having slaughtered all his people and enslaved their souls as his wraith-like monstrous minions, under the genuine delusion that he was doing them a favour.
- This very nearly happens to Harry, when he becomes the Dark Phoenix. Thankfully, The Power of Love was there in time to break the vicious circle.
- Simba in The Lion King Adventures. He goes from a sweet, mischievous cub to an evil, vicious killer. Initially, he's doing it to try finding a way to bring back the destroyed Pride Lands. However, it soon turns into a desire to become the greatest predator.
- Nightmare Phobia, the Big Bad of the Pony POV Series Chaos Verse, gets hit with this hard. Being that she was created from the remnants of Nightmare Moon's essence, she's naturally driven by a hatred of Celestia/the Mane Six and a desire to rule Equestria. However, as she grows more powerful, her memory becomes corroded, and she starts to forget why she hates the heroes, just that she does, and becomes obsessed with tormenting them, ultimately trying to kill them. And then, when she undergoes her One-Winged Angel transformation by absorbing the Shadows of Oblivion, she goes completely insane and delusional; she becomes convinced the heroes attacked her unprovoked, and that the only logical reaction to this is to kill everything in Equestria.
- The original Pony POV Series:
- There's definitely a a major example in the Dark World Series. It turns out that Discord has long been trapped in a "Groundhog Day" Loop by the true Big Bad, Nightmare Eclipse/Paradox, the original Dark World!Twilight, who went Nightmare in order to trap Discord in the time loops in order to torment him forever in an Ironic Hell. The thing is, her original intent before the corruption was to undo all the evil performed by Discord, but she lost sight of that in her obsession to gain revenge on him; by the time she's revealed, she's wiped out Dark World and everything in it several billion times, seeing everyone but Discord and Twilight as nothing but props, becoming far worse than Discord (who himself had a Heel Realization at some point) in the process.
- This is a running theme with Nightmares in general: most are driven with a genuinely benevolent or reasonable goal, but the power of a Nightmare comes with going insane. As a result, the majority of Nightmares become so lost in their goal they lose sight of the core attributes of it or begin doing things that they never would've done. There's a good reason the universal reaction a purified Nightmare has upon returning to sanity is My God, What Have I Done?.
- In Hope for the Heartless (in which the Horned King is brought back to life and half-forced on a seemingly impossible Redemption Quest), it's revealed that the lich (who has existed over a thousand years) has forgotten many things from his past in his centuries lasting quest to find the Black Cauldron in order to Take Over the World, viewing memories to be a distraction. He doesn't remember how to feel because his heart died out with the countless sins he committed, or the reason he wanted to conquer the world in the first place, or who he was before becoming Arawn's apprentice. However, over the many months he spends with his Morality Pet, Avalina, he gains some flashbacks from his past and starts feeling positive emotions again, even regaining his heart.
- In the backstory of Ages of Shadow, Jade absorbed the power of the Shadowkhan in order to save her family from a villain known as the Magus King. But the process was irreversible, and after some time she became corrupted and tried to Take Over the World herself, leading to her being sealed away. Now, countless centuries later, she's now lost large chunks of her memory.
- Downplayed in With This Ring. Paul merges with the Ophidian to save his friends and while Paulphidian is still largely benevolent and supports the good guys, they cannot feel any emotion besides Avarice and so they longer understand why they are so attached to them. They have completely forgotten what feeling emotions are like.
- BIONICLE 3: Web of Shadows: Roodaka takes advantage of Toa Vakama's mutation into a bestial state and promises to make him the ruler of his former city. After a while, Vakama abandons the "Toa" part of his name (meaning "hero") and forgets why he and his team even came to the city: to rescue its Matoran population. Later still, he becomes Roodaka's pawn and turns into an animalistic underling, until one of his team members hits a nerve in the deepest part of his sense of duty and reminds him of their quest.
- The Lion King II: Simba's Pride: Upon Nuka's death, it's implied by Zira's reaction to it that she did genuinely love her kids, but is so obsessed with getting revenge on Simba for Scar's demise that she's lost sight of that.
- Star Wars: Anakin Skywalker himself from the prequel trilogy joins the Dark Side in Revenge of the Sith in order to save his wife from dying. Then he starts killing younglings and things go downhill from there. When he force-chokes Padmé Amidala, the wife in question, because he believes she's turned against him, it solidifies him into this trope.
- Star Trek: Generations. Soran. He lost his family to the Borg, and eventually ended up killing hundreds of millions of beings just to enter The Nexus. When Picard tries to use the memory of his family to convince Soran to stop his plan, he pauses and looks distraught... then smirks and replies, "Nice try." This, implying that he knows this trope is in effect and just doesn't care, makes him less sympathetic.
- The titular character of We Need to Talk About Kevin invokes this:
Eva: I want you to tell me...why.Kevin: I used to think I knew. Now, I'm not so sure.
- Affects all the kids in Animorphs to some degree. Rachel in particular, as she can't remember what she was like before the war started.
- The Wheel of Time has an odd example in protagonist Rand Al'Thor. After he channels the True Power, which is derived directly from the Dark One (the series' Ultimate Evil), he undergoes an immediate personality change, becoming incredibly paranoid, very harsh toward anyone who he perceives as threatening or even disagreeing with him, and abandoning his previous refusal to kill or harm a woman. However, the extraordinarily stressful circumstances surrounding his use of the True Power could be considered explanation enough without resorting to this trope — he had been undergoing Sanity Slippage for upwards of six books — and it may even be considered a subversion considering that he pulls a complete 180 back to the side of good after an epiphany later in the same book.
It's outright stated by Rand that the reason he... freezes on the inside is because now he has (against his will, but that doesn't seem to matter to him) lucidly attempted to murder (one of) the woman he loves. The scenario is laid out in this passage(probably to preemptively avert people thinking it was the True Power):Or..no. That numbness had nothing to do with the power he'd held. He turned around, looking down at Min, who coughed quietly and rubbed her neck. She looked up at him, and seemed afraid. He doubted that she would ever see him the same way again.
He had been wrong; there had indeed been something more that Semirhage could do to him. He had felt himself killing one he loved clearly. Before, when he'd done it as Lews Therin, he had been mad and unable to control himself. He could barely remember slaying Ilyena, as if through a clouded dream. He'd realized what he had done only after Ishamael had awakened him.
Finally, now, he knew precisely what it was like to watch as he killed those he loved.
- Severus Snape from Harry Potter is a good example. He started out as a socially awkward but well-meaning young boy who desperately craved friendship, but due to his increasing bitterness over being bullied by James Potter and his gang, as well as the influence of his highly prejudiced friends, he began to become more accepting of Dark Magic and eventually called Lily Evans (the only person who truly cared for him) a mudblood, alienating her from him, while (according to Word of God) his desperate desire for friends led him to become a Death Eater.
- The Dresden Files: This is the reason why the punishment for breaking the Laws of Magic is always death- Black Magic is inherently corrupting and addictive, so by the time the White Council has found out about a warlock they're probably hip-deep in this trope and zooming towards the Moral Event Horizon if they haven't passed it already.
- The FBI agents in who became hexenwulfen to take down Marcone are described in these terms, their original goal buried under the evil influence of the belts. Harry briefly experiences it for himself when he uses the belt just once.
- Also many of the Denarians.
- In Changes, after Harry takes on the mantle of the Winter Knight, Sanya tells him there is always a way back to the light so long as he doesn't forget what he was fighting for.
- In J. R. R. Tolkien's The Lord of the Rings trying to use evil methods to accomplish noble ends always makes those who choose it eventually forget their original, good, purposes:
- Gandalf states that he would begin using the Ring out of pity and a desire to do good but that it would eventually make him another Dark Lord.
- During the Council of Elrond, Elrond states that "if any of the Wise should with this Ring overthrow the Lord of Mordor, using his own arts, he would then set himself on Sauron's throne, and yet another Dark Lord would appear. And that is another reason why the Ring should be destroyed: as long as it is in the world it will be a danger even to the Wise. For nothing is evil in the beginning. Even Sauron was not so."
- Saruman in studying Sauron and Ring lore came to envy him. He forgot his purpose in opposing Sauron and began imitating him, trying to set himself up as a dark lord.
- When Frodo offers her the Ring Galadriel says that she would become a new Queen, not dark but beautiful and terrible and that "All shall love me and despair!" but she resists the temptation to take it.
- The Ringwraiths are said to have all originally sought knowledge, and immortality, and used their rings to become great kings and sorcerers, but eventually they became completely dominated by Sauron's deceptions, enslaved to their rings, and were just extensions of his own will with no desires of their own.
- This is even the case for Melkor/Morgoth. He started out just wanting to create something of his own, and devolved into wanting to destroy everything he personally hadn't created.
- Sauron, originally Morgoth's lieutenant, originally felt the world would be a better place with the imposition of his order. Eventually he just wanted the power that came with ruling.
- Jacen Solo in Legacy of the Force (part of Star Wars Legends) ends up this way as a result of the philosophical growth he goes through that's necessary to save the galaxy without committing genocide. Twice invoked, as another reason he started down that path was because of a vision in which he and Luke fight to the death. Naturally, by the end he no longer cares if Luke dies.
- In the Star Trek: The Next Generation novel Captain's Honor, this happens to Lucius Sejanus, who, starting as someone merely wanting glory, tries to start the war with M'Dok and is willing to destroy the Enterprise for it.
- In Guy Gavriel Kay's Tigana, a woman infiltrated a court in order to kill the king. But she worked so hard at the infiltration that when she actually got a chance to kill him, she saved his life, and after that she actively helped him.
- In John Marco's Tyrants and Kings trilogy, this applies to Emperor Arkus and all his top lieutenants. In the beginning of his Black Renaissance they signed on to establish order and civilization on a continent wracked with war and devastation. But decades of conquest and immortality has corrupted them all, to the point everyone realizes they are irredeemable.
- In, Extreme Prejudice, a story in Jerry Pournelle's "High Justice" universe, an assassin is sent to an underwater research community to kill a defector. Over the course of the story, he realizes the defector has built a happy, productive life for himself and his family — and this just makes the assassin hate the defector even more; he has sacrificed any and all hope for a normal life in order to carry out his missions. It's when he discovers the reason for the defection that he realizes the trope is in effect; the President ordered the defector to kill his Solicitor General just as he began investigating his office for Mafia ties. He defected instead of killing a cop to protect a crooked politician — and warned the cop first.
Defector: You cant admit you're wrong. Youve spilled too much blood for the cause. Admit you're wrong and you're a monster. I know. I know.
Assassin: (In my business you do your job and that's all. ...you can't question your orders. If the people at the top don't know what they're doing, if it isn't worth it, what are you? A god-damned hired killer, a criminal, and I'm not that, I'm a patriot. A soldier.)
- So the assassin lets the defector escape. It's implied that the assassin also lets himself be eaten by sharks, as he feels a sick kinship with how they, "Come to blood from miles away. Eat each other. Stupid, single-minded killers." "The difference between a shark and an assassin is a matter of motivation."
- The Inquisitors of Disney's Star Wars have been revealed to have been all former Jedi, most of whom were padawans that were captured, tortured and brainwashed into serving the very people that destroyed their lives, their homes, their friends, their mentors, their Order, and the Republic.
- Bandora from Kyoryu Sentai Zyuranger turned evil after her son was killed by a Tyrannosaurus. Eventually she went mad with power and forgot all about this until her son came Back from the Dead. Her Power Rangers counterpart Rita Repulsa had a petty personality of pure evil, with her goal changed to her wanting to conquer the universe.
- Sam Winchester from Supernatural starts out trying to make the best of his Blessed with Suck-y demon powers by using them to exorcise demons, hoping to avenge his brother's death at the hands of Lilith. Even when Dean comes Back from the Dead, Sam remains obsessed with killing Lilith, thinking this will prevent the Apocalypse. With Ruby as his mentor, Sam eventually drinks demon blood to fuel his powers and gets addicted to the stuff. Dean stages a failed intervention and the brothers have a fight which ends in Sam trying to choke Dean and walking out on him. Sam then lets Ruby bleed a possessed woman to gain enough power to finally kill Lilith, knowing this should permanently alter him but expecting to die in the attempt, only to learn that Lilith's death was the final seal which had to be broken to free Lucifer.
- Charmed: Shortly after being freed of his original demon side, Cole was involuntarily possessed by the power of the Source of All Evil, and though he tried to resist for a time, it quickly turned him into the new Source, specially after Phoebe vanquishes the demon that would've taken the Source's essence for himself. After being vanquished, he came back from the dead and tried to use his powers to fight demons, though by this time he really should've known better. The sisters seem to give him the full measure of blame for his actions, even though Phoebe was partially to blame.
- Merlin (2008): Morgana. Averted in that it isn't the power in question that makes her evil and forget her friends, it's her actions with it. Her first act of sabotage against the tyrannical King Uther is to seek vengeance for the death of Guinevere's father. Fast-forward three seasons and Morgana has successfully overthrown Uther and taken over Camelot — but when Gwen reminds her that she too has suffered under Uther's reign, Morgana replies: "I had forgotten."
- Merlin (1998) Frik asks Queen Mab if her trick of letting Lancelot's wife Elaine know about Lancelot and Guinevere's affair wasn't unworthy of them. Mordred asks what "unworthy" means, and his mother Morgan has forgotten.
- Inverted and Subverted in Once Upon a Time. It's the evil characters who are shown to remember everything, while the "good" characters have forgotten their identities.
- And even once the curse is broken, the evil characters are usually the ones who keep grudges, and whose past has the strongest influence on their actions. Good characters' motivations are all tied to present conflicts, while the evil characters' motivations are all tied to things that already happened. The Evil Queen hates Snow White for something she did as a child, Rumplestilskin's life all ties back to his history with his father and his own son, and the Wicked Witch's whole motivation is to go back in time to Set Right What Once Went Wrong. Evil people are all hung up on the past, good people all focus only in the present, and when a character falls from grace (eg: Snow White's murder of Cora), it is because they let past events lead their actions (eg: All the evil things Cora had done up to that point).
- The long-term arc of Walter White on Breaking Bad, with "the Dark Side" being the dark side of chemistry — illicit drug production. He starts cooking meth to pay for his cancer treatment and provide for his family's future after he's gone. As time goes on, his justification gets more distant and less believable; he begins to start stealing, murdering and even poisoning a kid to get what he wants — which is to keep cooking and making money, even though his cancer is in remission and he's made more money than his wife knows what to do with. No wonder creator Vince Gilligan calls the series "How Mr. Chips became Scarface." It's only in the series finale that he admits that the real reason he kept cooking meth was because he enjoyed finally being successful at something.
- Mitsuzane suffers badly from this in Kamen Rider Gaim. He even references it.
Why... why can't I remember...? Why can't I remember what your smile was like? What was I trying to protect? What the heck was I fighting for?
- Some interpretations of The Wendigo curse involve people eating human flesh out of desperation because of the cold winter or famine, but as time goes on they start to crave more human flesh until they turn into a Wendigo and start eating people just because it tastes good.
- Par for the course when a performer pulls a FaceHeel Turn in Professional Wrestling; invariably, the new heel performer's original reason to turn heel is entirely justifiable based on something that had recently happened to them. Within a month of the turn, however, Motive Decay sets in and they end up generically doing things For the Evulz.
- Many Chaos Space Marines in Warhammer 40,000 originally turned to Chaos because of some noble goal (or for self-preservation); but after thousands of years in the service of the Great Gods of Chaos, they have usually either forgotten all about it or use it as a flimsy justification for the atrocities they commit. Then again, many of the factions of WH40K who haven't fallen to the side of Chaos still uses almost identical justifications to commit horrible atrocities.
- All of the surviving Chaos Primarchs. Magnus once wanted to spread knowledge, now his main desire is payback against the Imperium and especially the Space Wolves. Perturabo wanted to be respected; now he lurks on a Daemon World, the prisoner of his own resentment. Angron wanted revenge for the deaths of the only family he ever knew; now the party responsible has been effectively dead for ten millennia, but Angron doesn't care because he lives solely for killing. Mortarion once sought to end the threat of psykers; now he only wants dominion. Fulgrim's quest for perfection - and his Legion's - became a non-stop pursuit of excess. Lorgar is about the only one to remain reasonably clear, since his motivation before and after his fall to Chaos was the spread of faith; all that changed was faith in what. The remaining three are Horus (Deader Than Dead), Konrad Curze (regular dead), and Alpharius Omegon(who the hell even knows at this pointAlpharius died so hard his twin felt it from lightyears away).
- At one point in Warhammer Fantasy Roleplay, the use of necromancy drove you insane, so that no matter why you started practicing it you ended up a would-be world conqueror with an army of the dead. This has been relaxed, with necromancy being dark, creepy, and mostly used by evil people, but it doesn't have to be.
- A straighter example would be the Chaos Warrior career chain in second edition. The descriptions for each class bluntly describe that, with every step up the chain, the character forgets more and more of their past, gradually creating a new identity for themselves until they suffer a total Death of Personality.
- In Exalted, becoming an Akuma can result in an unusually literal version of this. So, you've made a deal with the Yozis to get the power you need — this calls for a Faustian Rebellion, right? Except that the process of becoming an Akuma excises your ability to do anything, and allows the Yozis to rearrange the inside of your head until it's more to their liking.
- The Forgotten Realms goddess Shar holds both darkness and forgetfulness in her divine portfolio.
- In Demon: The Fallen, Demons are able to tap into their Torment and unleash terribly destructive powers against their enemies. Unfortunately, the Tormented powers they use as a weapon leave a stain on their soul that becomes progressively more difficult to remove.
- Sweeney Todd: The Demon Barber of Fleet Street: The title character's whole story-arc is motivation decay using this trope. By the time that he finally takes vengeance upon Judge Turpin, Sweeney has gone quite murderously insane, and is so far gone that he fails to recognize his own daughter and tries to kill her. He's only snapped out of it in the end by the realization that the beggar woman he killed just prior to killing Turpin was none other than his wife Lucy, who Sweeney had gone on this whole thing to avenge, because his partner Mrs. Lovett didn't tell him that Lucy was still alive.
- Most versions of Faust have him originally making his Deal with the Devil for at least somewhat decent reasons, but he didn't exactly follow through on them.
- In Dreamgirls, both the play and the film, the Dreamettes' manager Curtis' morally questionable actions in the first act were for the betterment of the group in the face of a racist system. Come the 70's, however, much of that racism has faded but he's still shady. At this point it's clear that his only motivation is money and being in control.
- Fire Emblem: The Blazing Blade
- This was the revelation about Nergal learned in chapter 19x2, the most difficult bonus chapter to receive in the game. It turns out that Nergal is the father of Nils and Ninian. His research of the dark arts was so that he could find a way to open the Dragon's Gate and be reunited with his children. But the dark magic so corrupted his mind that he forgot the second half of this motive and ended up concluding that the reason he wanted to open Dragon's Gate was so he could destroy the world with dragons For the Evulz.
- It's stated by at least two other sources that this is what happens to anyone who dabbles with dark magic for too long, though as Bramimond demonstrates, they won't necessarily turn evil afterwards. Although it still doesn't display a terribly good fate for those who work with elder magic, as Bramimond is shown to have no personality of its own, merely mimicking the mannerisms of whoever it is speaking to. The lack of gendered pronouns in the previous sentence is a result of this — Athos initially calls it a "he" before switching to genderless pronouns and explaining that the true Bramimond is long gone and that the being they were speaking to was little more than a living mirror.
- The other source happens to be Teodor, also in 19x2, who explicitly says that this has to happen to anyone who wants to be really good with dark magic. Yikes.
- Fire Emblem Awakening: Supports in SpotPass reveal this to be the case with Gangrel, the first Arc Villain of the game. Walhart's conquest of the continent of Valm worried Gangrel, causing him to start his own unification plan that involved uniting Plegia, Ferox, and Ylisse under himself, so that they could stand up to the Valmese Empire. The power he got from starting the war, and his self-imposed rush to unite the countries, went to his head and caused Motive Decay; by the time of the game's beginning, his only real motive is deliberately provoking open war with Ylisse for the hell of it.
- More often than not, this happens to people who use fel magic in Warcraft setting. Fel will slowly corrupt the user, turning them evil; in the end they're likely to join the world-destroying Burning Legion in order to get more power. Examples:
- Demonic or fel influence: Sargeras, Archimonde, Kil'Jaeden, Kael'Thas, Illidan Stormrage, essentially every warlock, fel orcs/elves, Azshara, Ner'zhul, Gul'Dan (he was a huge jerk anyway but he got worse) and several others.
- Sargeras was once The Paragon of Good itself, and is more of a case of He Who Fights Monsters when he realized all he was fighting for would be for nothing if (and in his eyes, when) the Void Lords successfully managed to corrupt a sleeping Titan and turn all of creation into a hellhole and his fellow Titans refused to help him, so he decided to purge all creation in fire and destruction in the faint hope the universe would start again better prepared to fight off the corruption of the Void Lords. In this case, demonic energy didn't turn him evil so much as change his appearance to reflect his own new nature.
- Gul'dan didn't have any noble goals in turning to the fel; he was just a powerhungry jerk who sold out his race to obtain that power, making him one of the few true villains in the setting who has absolutely no sympathetic qualities to him. His teacher Ner'zhul, on the the other hand, did turn to the fel to help his people, but also had certain selfish motivations, and he eventually loses his redeeming qualities as well.
- Illidan is...complicated. He originally turned to the fel partially to fight the Burning Legion during the War of the Ancients, but also because of jealousy towards his brother Malfurion and a desire to be the hero. He's done both horrible and good things with that power ever since, and he's been on both sides of the conflict and his own side; heck, he even started an order dedicated to using fel magic to kill demons with his demon hunters. These days, he's a questionably-sane Wild Card who's more inclined to kill demons than Azeroth's people, but pretty much everybody's still wary of him.
- Necromancy: Arthas, Kel'Thuzad, Nerzhul (plus fel stuff and being selfish to start with) and all his liches like Ras Frostwhisper, some of the more prominent Death Knights...
- Moral of the story: using dark magics does not turn out well for whatever it was you wanted to do in the first place. Apparently, it's only in the RPG you find uncorrupt warlocks or necromancers, even though there's now a friendly (but still fairly evil) death knight faction. None of the playable or friendly warlocks in-game seem all that bad. Relatively speaking, anyway.
- Prince Arthas, particularly in Warcraft III (World of Warcraft arguably doesn't count because the Lich King had mind melded with him). Arthas started out wanting to stop the undead scourge; then he wanted to kill Mal'Ganis to avenge the people who had been killed or turned by the undead. Then he becomes a Death Knight, travels back to civilization and immediately starts killing people. While the descent started a bit earlier, the Artifact of Doom Frostmourne (and its corrupting influence) was the part that made him completely forget that he ever cared about keeping people alive in the first place.
It still counts in World of Warcraft as, for the better part of the time between his ascent to the Frozen Throne and Wrath of the Lich King, Arthas has been in a torpor reflecting on his actions in his rise to power, as detailed in the book Arthas: Rise of the Lich King. By the end, Arthas comes to terms with using The Dark Side, and actually assimilates Ner'Zhul, taking his power and becoming the sole force of will behind the Lich King.
- Demonic or fel influence: Sargeras, Archimonde, Kil'Jaeden, Kael'Thas, Illidan Stormrage, essentially every warlock, fel orcs/elves, Azshara, Ner'zhul, Gul'Dan (he was a huge jerk anyway but he got worse) and several others.
- Knights of the Old Republic
- Yuthura Ban. If you ask her about her Backstory, she'll admit to first becoming a Sith because she had spent her adolescence as a slave to the Hutts and she wanted the power to free others who suffered like she had. She'll admit to being surprised at not having thought about that motive for years, and you can even call attention to the fact that Sith training runs counter to it (a full-fledged Sith wouldn't care). This is actually important for your final confrontation with her. She is one of the few people who'll accept the Last-Second Chance you give if you've shaken her faith previously.
- The sequel explains that this also happened to Revan.
- Malak started out as Revan's right-hand man and one of the Jedi who wanted to get out there and actually save the Republic. After bathing in the Dark Side, he betrayed his closest friend, then started trying to take over the Republic himself.
- In the City of Heroes back story, Vanessa DeVore used Giovanna Scaldi's Soul Jar mask to help fight off the Rikti. Unfortunately, the power — and Scaldi's evil influence — corrupted her, leading her to form the Carnival of Shadows enemy group. The effect was permanent, even after an attempt to free her from Scaldi's influence she didn't change at all.
- Wiegraff from Final Fantasy Tactics starts out as a battle-hardened yet genuinely well-intentioned rebel who develops a personal vendetta with Ramza after the latter kills the former's sister. His desire to avenge his sister's death is what leads him to become a knight in the Church. After a second failed battle against Ramza, Wiegraff makes a Deal with the Devil and becomes a Lucavi Demon. The last time we see him, for his final confrontation with Ramza, he explicitly states that he no longer cares about what happened to his sister, and just wants to cause as much pain and chaos as he can. The speech he gives is actually rather unnerving.
It's a even worse than it sounds: he didn't just decide one day to let a demon into his head. After the second battle he was bleeding out and about to die, cursing himself for being unable to avenge Miluda. Only then does the Zodiac stone offer him another chance by accepting it's power, and he takes it; he was never told the stone held a demon.)
- Case of the Lifestream, a Final Fantasy VII novella, shows that Sephiroth considers memories of his friends, appearance, and childhood "unimportant" compared with his contempt for humanity.
- Dragon Quest IV has this happen to Psaro the Manslayer, who starts out thinking Humans Are the Real Monsters due to how they treated his beloved Rose and sets out to ensure she'll never be hurt again... despite her constantly begging and pleading with him to stop. Things only get worse after she's murdered by human thugs, causing him to declare he'll simply Kill 'Em All. By the time the chosen ones confront him, he's used the perfected Secret of Evolution and completely lost his mind, declaring that all he remembers is his name and his hatred of humankind.
- Arakune in BlazBlue starts out as a Well-Intentioned Extremist version of scientist named Lotte Carmine, who wished to use the power of Boundary in order to achieve a discovery that would be beneficial to mankind in the long run... even at cost of a lot of human lives. He tapped onto it, and slowly turned into a man-eating Eldritch Abomination that couldn't remember anything except "Want to eat", especially the Azure Grimoire. He still remembered some of his best friends like Litchi, but that 'discovery beneficial to mankind' AKA his original intention? All forgotten.
- Kingdom Hearts:
- Riku goes through this in the first game. He starts out using the darkness because he wants to retrieve Kairi's heart. This eventually takes second place to his jealousy of Sora — after taking the Keyblade, losing the Keyblade and getting his ass kicked, he's willing to open himself completely to the darkness if it means he can be stronger than Sora. Big mistake.
- Completely averted with Terra in the prequel. Through most of the game he's very careful with his use of darkness, and never loses sight of his morals or priorities. Unfortunately, he ends up even worse off than Riku.
- Master Xehanort started out as a Well-Intentioned Extremist, believing that the universe is too full of light and that a perfect balance between light and darkness is needed; his plan to trigger a second Keyblade War was so he could acquire the power of Kingdom Hears and remake it in his image of balance. Of course, he's long since become Drunk on the Dark Side, and now just wants to cause an apocalypse For Science!. He does have a Heel Realization about it when he's finally defeated in the end, though.
- At the end of Birth By Sleep, certified paladin Aqua's Heroic Sacrifice trapped her in the Realm of Darkness, which she did deliberately so her friend could leave it. Ten years of wandering the darkness later, she (or what is left of her) has completely forgotten that and blames Mickey for "abandoning" her. In accordance with this trope, the instant she is magically purified with Light she recognizes the truth.
- Lucien in Fable II. He originally wanted to rebuild the Tattered Spire in order to revive his dead wife and daughter, and when he realizes that the protagonist is The Chosen One destined to stop him, he is visibly shaken when he has to kill you and your sister. However after decades of building the spire on the back of slave labor and dark magic, he's completely forgotten his original purpose for building it, and only wants the power. This is most obvious the second time he shoots you, in which he'll calmly state he just murdered your family in cold blood, and then kill you without a second glance.
- Logan from Fable III is a subversion. He never forgot what drove him to tyranny.
- The Legend of Zelda: The Wind Waker implies this was what happened to Ganondorf. He initially set out to conquer Hyrule because he envied its idyllic nature while his people lived in a harsh desert... but when he laid hands on the Triforce, seven years later all he'd changed is that now the rest of Hyrule is even worse than the desert while he chased the missing pieces of the Triforce. Skyward Sword actually gives an explanation for this- he's the incarnation of a God of Evil's hatred, so no matter what his starting goal, The Dark Side will eventually corrupt it into generic Evil Overlord-ism.
- Mass Effect
The Illusive Man: Cerberus is humanity!
- The Illusive Man. He begins the game as a reasonable, if morally questionable, authority figure, but progressively gets more questionable, at least from the perspective of a Paragon Shepard..
The Illusive Man: Don't turn your back on me Shepard, I made you, I brought you back from the dead!
- This is made worse in the third game where it is revealed that the Illusive Man and all of Cerberus got indoctrinated into serving the Reapers. For the Illusive Man anyway, this shows up so subtly that he fails to see it even when he starts huskifying his own men and not even caring when the organization he built from the ground up got burnt to the ground. Admittedly, he does see it at the end if you managed to convince him to kill himself.
- Played with, however, in that The Illusive man is indoctrinated. Shepard would have turned him around, but he's incapable of accepting.
The Illusive Man: (conflicted) Your idealism is... admirable, Shepard.
- Played with, however, in that The Illusive man is indoctrinated. Shepard would have turned him around, but he's incapable of accepting.
- Daisy Fitzroy, the leader of the Vox Populi in BioShock Infinite, ends up thinking she has to kill children of upper class parents on the chance they become Corrupt Corporate Executives. Burial At Sea subverts this, it shows that this was a Batman Gambit she was doing to get Elizabeth to kill her on behalf of the Lucene twins. She had no intentions of harming Fink's son for real
- The Templars in the Assassin's Creed series. While modern society wouldn't begin to agree with their ideas of a plutocracy, their overall goal of creating a utopia where there's no more war and suffering isn't a bad one. But they never reach that point, even by the modern age; the technologies they find combined with the power they already wield by 2012 in the AC Universe would be more than enough to sit down and establish their utopia, or at least parts of it. Instead, even in 2012, they simply keep hoarding more and more power, killing more and more people to get more power, and never reach the point where they feel they should finally sit down and install their utopia instead of killing the Assassins or doing the two aforementioned goals. This is in part due to the continued interference by the Assassins, who are ideologically opposed to them, but their Well-Intentioned Extremist stance on Utopia Justifies the Means keeps falling flat on it's face when presented to us, the players, because they never even try to actually install their utopia in the first place, instead feeling they just never have enough control of the populace.
- This gets explained in III: the order as a whole has been carefully manipulated over the ages to provide a framework that instills a sense of... misguidance in its members, leading to their desire for control in all things, leading to acceptance of domination (enslaving others) and subjugation (being enslaved) themselves. You barely see any of the leaders perform a Klingon Promotion; they toss away the lives of innocents to get where they are instead of taking a page from the assassins and just killing their way to the top. This usually results in the few individual Templars too stupid or too egotistic to be brainwashed taking control of the whole damn thing, which is the main reason why the order is so fucked up. The whole point of this was so that, in the event that an Ubermensch took command, that person would rule the entire planet unopposed and well-defended. That person is Juno, one of the First Civilization survivors, who brain uploaded herself in a bid to take revenge on humanity by using them to warp reality.
- Gungrave's Harry MacDowell. Played to full, tear-jerking effect.
- In The Elder Scrolls III: Morrowind, Big Bad Physical God Dagoth Ur may have this trope going on depending on which version of backstory events you choose to believe. According to one account of his history, he only started using the power of the Heart of Lorkhan in order to protect it from being abused by the Tribunal; but it corrupted him and he became worse than they ever were.
- Middle-earth: Shadow of War: Talion and Celebrimbor use Wraith Artes to mind-control Uruks and neutralize Sauron. This slowly erodes their humanity as they die repeatedly, performing war crime after war crime in between. By the end, Celebrimbor has developed full megalomania and intends to take over as a Bright Lord, enslaving man and orc for the sake of the elves. Talion eventually breaks after decades of resisting the corruption of necromancy, but manages to get out thanks to the efforts of the Fellowship of the Ring, only to completely forget his family and become a wandering spirit who forgot everything he originally stood for (signified by his abandonment of the very weapons he and Celebrimbor forged in the name of his family).
- In Devil May Cry 5, Big Bad Urizen is so obsessed with obtaining power at any cost that he no longer remembers his past as Vergil or the reason he wanted power in the first place. In fact, this lack of motivation is why in the end he lost to Dante who actually has something he wants to protect.
- Discussed in Metal Gear Solid V: The Phantom Pain:
Skull Face: You think that hatred might someday replace the pain. But it never goes away. It makes a man hideous, inside and out.
- Zouken Matou from Fate/stay night. He originally began his hunt for the Holy Grail as a way to honor the woman he loved, Justica Von Einzbern. But as he grew older and began turning to more and more twisted magic to keep himself alive long enough to accomplish this, his soul began to rot and his mind went with it, leaving him a corrupt, ruthless madman obsessed with immortality.
- In Ace Attorney, Miles Edgeworth start out as a child with the idyllic view that defense attorneys protect the innocent (a view shaped by his father, a defense attorney who was the closest the series had to a Big Good). Following said father's murder and an upbringing by his murderer, prosecutor Manfred Von Karma, Edgeworth forgot his childhood beliefs and began to view defense attorneys as obstacles to be crushed on his way to keeping a perfect record of guilty verdicts. As the series goes on, he remains a prosecutor but remembers what he originally knew law was about, focusing his career on uncovering the truth in every case.
- Vaarsuvius goes through this in The Order of the Stick. He/she accepts a Deal with the Devil to gain the power needed to save his/her family from a dragon with a grudge. After killing the dragon, he/she zombifies what's left of her so he can commit genocide on her extended family while she watches. And then, his/her spouse insists she relinquish the power right now, Vaarsuvius refuses and leaves to "fix everything", and ultimately loses the power anyway in an ill-fated attack on the Big Bad. The worst part? The Fiend Corporation who gave them this power outright lied to them about the corrupting, mind-enslaving effects of black magic; the only influence it has is talking to bad people who lost their superpowers. Everything Vaarsuvius did was done of their own free will.
- Trace from TwoKinds tried to resurrect his dead wife before black magic poisoning turned him into a power-hungry dictator. Or at least, that's what a character told him...
- King Lewstrom, the lich necromancer of Yet Another Fantasy Gamer Comic, is revealed to be this. His backstory is eventually told, and involves him turning to a dark goddess in order to gain revenge on those who killed his family. Centuries later he is a lich and remembers none of this. Upon finding out, he asks his goddess why she never held up her end of the bargain. She claims she did; he out-"lived" all of his enemies.
- It's explicitly mentioned in Tower of God that, on their way to the top, only a rare few remember the reason they originally wanted to climb the tower for. Most forget it amidst the power and riches they encounter, and don't realise it until much later, if at all.
- While only "evil" in a tongue-in-cheek way, Artefact of Sankaku Complex was not always a depraved anime-yellow-journalist-slash-pornoblogger. His earlier writings were more gushing and humble in tone (in contrasts with his signature witty Deadpan Snarker style), recommending Yotsuba&! for learning Japanese, as well as promoting artists, works, or stores he likes. Compare old titles like "Yotsuba-to! Ideal manga for learning Japanese?" to the typical "Ichiban Ushiro no Dai-Maoh Bukkake Milking Anime" common nowadays.
- Los Magnificos, the heroic defenders of Mexico City in the Global Guardians PBEM Universe, made a deal with the Garodillos crime family to turn a blind eye to certain crimes in exchange for the crime family's assistance in policing the barrios they controlled of its more extreme elements, because the problems faced by those areas were too much for the heroes to handle on their own. And it worked, for a while. Sure, there was still drug-trafficking, but murders and rapes went down. Eventually, however, the members of Los Magnificos were little more than a superhuman enforcement arm for the Mexican mafia.
- As Kasei's Motive Rant in We Are Our Avatars reveals, he was already willing to do heinous things in the name of science, but he soon began to enjoy them more and more until he became little more than a sadist, doing research less to discover things as to gain the power to stop anyone from challenging him.
- Star vs. the Forces of Evil
- Toffee eventually shapes himself as the Big Bad of the series. Word of God compares him to Magneto, as someone who is trying to do good in "the wrong way". This might explain his initial revolt, but by present time he seems to be just a Manipulative Bastard with no end goal in mind.
- Eventually, this trope is downplayed. Toffee's goal of "good" is in ridding the realm of magic as he views it as dangerous and something Star comes to agree with. On the other hand, later episodes show he was always a jerk (even back as an adolescent, being reminscing of a punk gang leader) and that his complaints seem dishonest since his race, the Septarians, are invulnerable with a strong Healing Factor that only magic can harm. Even most other monsters were afraid of his kind, making his own goals of "good" seem hypocritical when compared to his race's own talents and him beign afraid of something that can actually hurt him.
- Pinky and the Brain
- Sigmund Freud determines through hypnotism that Brain's desire to rule the world was actually created in him subversively and accidentally by the scientists at his lab. What Brain really wanted was to go back to his family, who lived in a can with a picture of the world on it. But his mind was warped by the experimentation to the point that all he could remember was his desire for that image of the world. Brain considers the possibility, but ultimately dismisses it and goes on as planned.
- Also, a combination of Well-Intentioned Extremist and Aesop Amnesia add up to this trope in the episode where Brain turns away from megalomania and vows to spend his life helping the victims of his obsession (first and foremost, Pinky). Despite Brain's best efforts to help prepare him, Pinky (as a tiny, dumb mouse) gets trampled and abused in the real world, prompting Brain to swear that he WILL take over the world, in order to make it a better place that would be fair and kind to people like Pinky. This motivation never comes up again, and Brain is back to abusing Pinky at will in the next episode.
- Star Wars: The Clone Wars
- Asajj Ventress spent many of her early years training under a Jedi who treated her with respect and seemed like her happiest days of her life. Then her master gets killed, she goes on a Roaring Rampage of Revenge, Dooku recruits her, she is now his apprentice and she goes around killing Jedi in revenge for not being there for her master.
- She and the Nightsisters did this with Savage Opress. They used some sort of Dark Magic, to transform Savage into a monstrous giant, who killed his brother when Asajj ordered him to do so. Earlier he became Asajj's slave willingly, in exchange for said brother's freedom.
- Star Wars Rebels
- According to Word of God, The Inquisitor was one of the Jedi Sentinels who overheard Bariss Offee's proclamation of the failures of the Jedi Order and her words planted the seed of doubt in his heart. Unfortunately, by becoming an Inquisitor, he has joined with the very people that caused the Jedi and Republic's fall from grace.
- Teenage Mutant Ninja Turtles (2012): Shredder's love for Tang Shen, and her choosing Splinter over him, is his primary reason for turning evil. By the time of "Requiem," he's fallen so far that he no longer seems to care about Tang Shen at all, having absolutely zero qualms against mocking Shen's death to Karai's face by declaring that it's fitting that Karai will die in the same manner as her mother.
- More recent Transformers storylines have this happening to Megatron. He usually gets his start as an idealistic miner-turned-gladiator chaffing under the Fantastic Caste System that's been crippling Cybertron, starting protests against the corrupt Autobot leadership and forming a friendship with Orion Pax, an Autobot who believes that his cause is just. Unfortunately, his own growing discontent with the system, his bloodlust, and his desire for power and the right to lead Cybertron into a new Golden Age eventually makes him more radical and turns him into someone just as bad as the ones he was fighting against as he goes to war with Orion, now his mortal enemy Optimus Prime. By the time of the present day and their conflict gets to Earth, Cybertron is a war-torn wreck and Megatron is a warmonger who will settle for nothing else than total domination of the universe, freedom the farthest thing from his mind.
- The second season of Voltron: Legendary Defender, reveal this to be the backstory of Zarkon and Witch Haggar. Zarkon was a King who was desperately fighting to preserve his dying planet. Witch Haggar was a Altean scientist passionate about her work. The two of them meet and fall in love. Sometime later they discover a rare comet made from a very powerful ore. Zarkon believing the ore is the answer to his planet's problems, and Haggar being fascinated by its unlimited power to the point of obsession, eventually drove both of them into a downward spiral until both of them Came Back Wrong.