Jane: Don't fall for it, Jack. 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 what it was we wanted it for in the first place.
— Head Office
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 have to use The Dark Side. Using the power itself is a technically neutral action, and in theory, it should be possible to use it to achieve the goal in question. But The Dark Side being The Dark Side, it doesn't work like that. By its very nature, the power will somehow effect an alignment change in the character, and they will be a completely different person than they were before they tried to use the power. They may or may not realize it, and may keep going even if they do. It can be a gradual process, as It Gets Easier. (In video games, this is usually regarded as The Corruption.)
This is a fairly common method of making an evil character sympathetic — but not too sympathetic. 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. Often the justification for Motive Decay.
This trope is usually pretty sad, actually, since in a sense it makes the evil character themselves a victim of evil.
May be the lead up from Kick the Morality Pet. A justification for Jumping Off the Slippery Slope.
Compare to He Who Fights Monsters. Related to Dark Is Evil and Bad Powers, Bad People.
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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.
Death Note's Light Yagami originally had good intentions, but as he got more and more drunk on power he lost sight of them in favor of taking over the world. Alternately, he had superego-compatible justifications, which fell by the wayside as he became more and more used to the safety of anonymity and the fact that people who liked Kira liked him for his power more than anything, and that sparring with L was actually a lot more interesting than taking out the trash. Either way his access to dark side powers seduced him to casual evil without his noticing.
Light's first thought when testing the Death Note was to use it on a group of bullying classmates. He only decided not to because that might lead the police to him. These are not the thoughts of a normal, balanced individual. Moreover, his first attempt to kill L was motivated by rage — rage at being called evil. He also killed investigators whose job it was to stop crime. Though he may have reason to kill those trying to stop them, he was disturbingly satisfied that he outwitted and forced a broken detective to hang herself. The Death Note's effects on Light's sanity became more obvious when Light gave his Death Note away and lost all of his memories. He turned out to be nice and fully convinced that Kira was evil and had to be stopped.
Although it didn't take long for this to happen, and he doesn't seem to have had far to fall. By the end of the first episode he's already explicitly telling Ryuk that his goal is to rule over the world like a god.
Sasuke is a poster child for this. While he was always firstly driven by his wish to kill his brother, 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 not only he was willing to wound Rangiku to accomplish that, it seems would have continued Aizen's plan to destroy Karakura Town if he had managed to kill him. 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 Soukou No Strain. 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 IDW's Transformers 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.
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.
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.
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.
In the miniseries Merlin, 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.
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 Wordof God) his desperate desire for friends led him to become a Death Eater.
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, Gandalf refuses to take the Ring for this reason. During the Council of Elrond, Elrond states that anyone using the Ring to overthrow Sauron would only create a new Dark Lord in his place. This is even implied to happen to Big Bad Morgoth and his lieutenant Sauron: The former started out just wanting to create something of his own, and devolved into wanting to kill everything. The latter felt the world would be a better place with the imposition of his order, and then just wanted the power that came with ruling. As you can see, this is a major theme in Tolkien's works likely due to his own religious views.
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.
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. Changed in the english version 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.
Used rather badly on Charmed with Cole. Shortly after being freed of his original demon side, he 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: 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."
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.
Par for the course when a performer pulls a Face-Heel 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 commithorribleatrocities.
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.
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.
Goethe's Faust, on the other hand, averts this; he is ultimately saved because he doesn't fall into this trope — despite satisfying the Exact Words of the contract that will damn him, he never stops searching for transcendent truth, which was the real reason he made the bargain in the first place.
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.
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'talwaysturn 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.
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 all the pain and suffering and hatred he had seen across the universe got to be too much for him and he decided the better solution was to just burn it all. In this case, his motive didn't drive him to evil before decaying, it decayed and then drove him to evil.
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.
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.
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.
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.
Riku goes through this in the first Kingdom Hearts 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.
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 tokill 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 possessed by the death-curse of an ancient demon, so no matter what his starting goal, The Dark Sidewill eventually corrupt it into generic Evil Overlord-ism.
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: Cerberus is humanity!
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.
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. The Tears that Elizabeth and Booker pass through in order to make that part of Daisy's history doesn't seem to help.
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 Phoenix Wright: 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 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 ("Darth V") goes through this in The Order of the Stick. S/he turns to the dark side to save his/her family, but after that's done s/he stays that way just to torture his/her enemy some more.
So far averted with Durkon. He may have changed alignments, but even as a vampire he's still on board with saving the world.
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.
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.
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.
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, and she is now one of the most effective Jedi-killers in the Galaxy.
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.