The Dark Side Will Make You Forget

"And though I'll think of you, I guess Until the day I die, I think I miss you less and less As every day goes by, Johanna."

- Sweeney Todd: The Demon Barber of Fleet Street, "Johanna Quartet"

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 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.

Anime and Manga

 * In Magical Project S, Misao Amano was an evil magical girl aiming to ruin the balance between good and evil. When she transformed into a form concerned with expressing the negative, repressed parts of her personality, however, she simply did things For the Evulz and lost 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 almost immediately tried to kill people he knew  weren't criminals. His first attempt to kill L was motivated entirely by anger at L's taunting. I don't think he was ever anything but evil incarnate.
 * Gungrave's Harry MacDowell. Played to full, tear-jerking effect.
 * Sasuke from Naruto 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.
 * 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 from Bleach. It seems he originally started  Now over a century later
 * 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.
 * 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.

Film

 * Star Wars's Anakin Skywalker himself joined the Dark Side in order to save his wife from dying. Then he started killing younglings and things went downhill from there. When he force-choked Padme, it solidified 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 tried 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.
 * Well his plan would put him in the Nexus where he would be with his family for all eternity, in general reminding the villain of what they stand to gain/regain from their plot is not a good idea.
 * In the miniseries Merlin, Frick acts Queen Mab if her trick letting Lancelot's wife Elaine know about Lancelot and Guinevere's affair wasn't unworthy of them. Mordred acts what "unworthy means", and his mother Morgan has forgotten.

Literature

 * Wheel of Time has an odd example in protagonist Rand Al'Thor. After he, 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   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.
 * The FBI agents in The Dresden Files 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, 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 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 the latter felt the world would be a better place with the imposition of his order.
 * This recently happened to Jacen Solo in Legacy of the Force, part of the Star Wars Expanded Universe -- as a result of the philosophical growth he went through that was necessary to save the galaxy without committing genocide.
 * In the Star Trek TNG novel Captain's Honor, this happens to Lucius Sejanus, who, starting as someone merely wanting glory,
 * 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 every one realizes they are an irredeemable Complete Monster.

Live Action TV

 * 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
 * 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. 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.
 * 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.

Tabletop Games

 * Many Chaos Space Marines in Warhammer 40000 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.
 * At one point in Warhammer Fantasy 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.
 * 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.

Theater

 * 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, Sweeney has gone quite murderously insane, and is so far gone that . He's only snapped out of it in the end by.
 * 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.

Video Games
"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!"
 * In Fire Emblem, this was the revelation about Nergal learned in chapter 19x2, the most difficult bonus chapter to receive in the game.
 * 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  demonstrates, they won't always turn evil afterwards.
 * Although it still doesn't display a terribly good fate for those who work with elder magic, as 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  is long gone and that the being they were speaking to was little more than a living mirror.
 * 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 Complete Monsters in the setting as he has apsolutely 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,
 * Actually, it's worth pointing out that a rather large portion of these don't actually start using Fel/Necromantic power all that willingly, and those that do generally stick pretty closely to their original goals up until WoW flanderized every major character. Notably, Illidan became a half-demon to destroy Tichondrius and a rather large army, which he did, and then worked for Kil'Jaeden to kill Arthas, which he tried (and failed) to do, neither of which were particularly bad. Things just don't turn out well, because nothing in Warcraft 3 ever turned out well in the long term.
 * Knights of the Old Republic has 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).
 * The sequel explains that this also happened to.
 * 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 . After a second failed battle against Ramza, Wiegraff  . 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 little worse than it sounds;
 * 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.
 * 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.
 * Dragon Quest IV has this happen to Psaro the Manslayer, who starts out thinking Humans Are Bastards 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, causing him to declare he'll simply Kill'Em All. By the time the chosen ones confront him, he's used   and completely lost his mind, declaring that all he remembers is his name and his hatred of humankind.
 * Arakune in Blaz Blue 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 to 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.
 * Logan from Fable III is
 * 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 Side will eventually corrupt it into generic Evil Overlord-ism.
 * Arguably, the The Illusive Man from Mass Effect. 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..


 * This is made worse in the third game where

Web Original

 * 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.
 * 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...

Western Animation

 * In Pinky and The Brain  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.
 * In 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, 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.

Web Original

 * 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.