Subverted in Angel Beats!, where Tenshi manages to win out against dozens, if not hundreds of murderous versions of herself after all of them recombine into one person. Given her exhausted state when it happened, and Key's tendency to Kick the Dog, it was a close call.
Both are okay with that. Neither one is okay with the omnicidal maniac side taking over at the end of the manga, leading to a shoot the dog moment from Kouta.
In Animerica, this occurs to Kiyone, explaining his Magnificent Bastard actions in Season 2. The "evil" side attempts to do this again in the beginning of Season 3, but thankfully it gets destroyed for good this time.
Played straight and very moving and freaky in Trigun, especially the manga: Knives Millions's short transition from a hugely caring and even oversensitive cute boy to a Woobie who hides a lot of anguish and anger management issues from his loved ones to the friggin' biggest homicidal psycho in Gunsmoke's short history and the responsible of what's probably the biggest genocide in human history, period. The anime arguably oversimplifies his transformation and motivations, making his Freudian Excuse quite ludicrous.
The manga also suggests that Legato Bluesummers' original personality may have been pretty ordinary and that he used to have normal human needs, but that it soon got twisted beyond recognition by years of horrifying abuse in a small town plagued by criminality (and pedophiliac homos) in the middle of nowhere-in-Mordor and not even having a name, parents, or a past worth mentioning in his view.
In Saint Seiya, the Big Bad of the first major plot arc is Saga, Gemini Gold Saint, a saintly (forgive the pun) warrior in Athena's service, whose Split Personality is thoroughly and irredeemably evil. Naturally, it takes over for good, which results in the Sanctuary War, although Saga's good side still shows occasionally (if only to have an Heroic BSOD as his own actions.)
This is why Moka never takes off the rosary herself in Rosario + Vampire: the outer Moka would be replaced by the inner Moka forever. This doesn't stop her from trying it anyway.
Marik and Yami Marik from Yu-Gi-Oh!. The split personality was born when Marik's father carved his back. The pain was so severe that Marik created a second masochistic personality to cope with it. Yami Marik took control to kill his father a year later and intended to do away with the rest of the family, but he retreated back into Marik's subconscious due to his Morality Chain adoptive brother. Whenever Marik's brother is possibly fatally harmed, Yami Marik takes control.
In Bleach, this is what Ichigo's inner Hollow intends to do. On a few occasions he manages to briefly seize control of Ichigo's body, but Ichigo manages to regain control one way or another. His training by the Vizard was intended to help him prevent this from happening while also gaining control of the Hollow's powers.
This is also the basis for most Hollow transformations, with the original personality being largely destroyed in the process. Sora was temporarily able to regain control of his Hollow body after Ichigo shattered his mask.
Menos Grande are constantly in danger of this. A single Hollow dominates the body, with the rest constantly trying to take over. If the dominant Hollow is defeated, the Menos will become little better than an animal as the various personalities fight for control.
Inverted with Riff in Count Cain, who was given a nice personality in order to gain Cain's trust, which then faces off, and wins, against the original evil one.
Pretty Sammy quotes this trope to Misao which it encourages her to help her in getting rid of Ramia's evil influence over her.
Even if you stay as Pixy Misa forever, I'll still really love you, Misao!
In Psyren when Amamiya is shot Abyss, her other personality created from her suppressed emotions takes over momentarily.
Played with in Samurai Deeper Kyo. In the manga version, Demon Eyes Kyo is a legendary killer whose mind has been sealed inside the body of Mibu Kyoshiro, but occasionally rises to the surface. In the anime version, Demon Eyes Kyo is the dark side of Kyo's personality that escaped and became an Enemy Without, then was reabsorbed back into Kyo. In both versions, Kyo eventually assumes full-time control of Kyoshiro's body.
Although Naraku already dominates Onigumo, he plans on removing his heart from him since his love for Kikyo prevents him from hurting her.
A more classic example comes from Inuyasha himself: whenever in the presence of a powerful demonic aura and/or separated from the Tetsusaiga, his demon blood takes him over and he transforms into his fully demonic form, then proceeds to wantonly murder anyone nearby. It takes an increasingly large amount of effort to bring him back to normal each time.
One could argue this is what happened to King Bradley/ Wrath in Full Metal Alchemist. Bradley's body's original owner was bound to a Philospher's Stone and waged a war with the various personalities trapped inside the Stone until the current Bradley became dominant. Even he's not sure whether he's the original test subject or some other personality.
Averted with Ling/ Greed, which started off this way but ended up as more a case of Symbiotic Possession.
Shiro in Deadman Wonderland seems to have had her violent side take over in recent chapters. Chapter 55, however, reveals that this is Averted, as her supposed Split Personality never existed in the first place. She is incredibly jealous of Ganta for being able to live a comfortable life she never had as a result of being his replacement and was doing everything in her power to make sure he was strong enough to kill her.
In Frank Miller'sThe Dark Knight Returns, Harvey Dent has received surgery to repair his face's damaged half, but it has the opposite effect on his mind - Two-Face is now entirely in control.
They were right about each half of his face representing one of his personalities... they were just wrong about which side represented which. Oops.
There's a Spider-Man arc involving Symbiote knockoffs. Venom, of course, always refers to himself as 'We' (as in, Eddie Brock and his Symbiote), so when one of the symbiotes stops talking in plural, Spidey knows that she's all gone - the Symbiote has taken over her mind.
An interesting conclusion of this is that the very thing that makes Venom sound like a raving lunatic is actually an indication of his remaining sanity - or even a psychic crutch necessary for him to retain his identity.
Carnage called himself "I" as well, although in that case it was Cletus Kasady, who was already a psychotic killer, who took over the symbiote's mind. The explanation that was recently given is that this was because the symbiote was just a baby when it first bonded with Kasady. Or, Kasady was just SO CRAZY, that he and the symbiote both wanted the same thing.
While it wasn't made as big a deal of as the animated version, it used to be that the Goblin Serum gave Norman Osborn a Split Personality, and would periodically forget he had a Superpowered Evil Side and go back to being an industrialist. Since he came Back from the Dead, it seems to be accepted that he's the Goblin all the time. Though during his time on Thunderbolts and Dark Reign, it seems the split personality explanation is back. Subverted in that Osborn is still an evil jerk, the Goblin is just his darker side.
A horribly tragic case occurred to Dan Turpin during Final Crisis. To be fair it skirts between this and Demonic Possession, but after visiting Bludhaven in search for missing children, Danny starts getting blackouts and finds he can't control himself after a certain point. He then discovers with horror that he is being possessed and/or erased by Darkseid. After he is completely possessed, he is poisoned by Batman, nearly killed by the Black Racer, and finally and utterly destroyed by Superman.
The final issue implies, but never outright states, that it was only Darkseid who got destroyed and Turpin was ultimately freed. It's probably a moot point now that DC has reset the universe again.
Inverted, to some extent, with the Hulk's son Skaar, where the weaker, more vulnerable personality ('puny' Skaar) has recently managed to escape Skaar's suppression of him in their shared mind.
During the Spider-Man storyline The Gauntlet, Curt Connors' worst fears are realized when he loses a battle of wills with the Lizard part of his psyche. The Lizard kills and eats Curt's son Billy, and "Curt" essentially dies due to the trauma, leaving only The Lizard.
This was Bloody Mary / Typhoid Mary's (a villainess most often found in Daredevil) entire schtick. She had rather impressive psychic and telekinetic powers, but she also had a horrible case of multiple personality disorder; namely, at least nine of them. Two of them (the aforementioned Bloody/Typhoid Marys) tended to take over more often than not, and there have been at least two instances where one or both have "permanently" taken over only for later events to unlock her original (non-powered) personality once more.
This has happened to Jason Blood from time to time, depending on who's writing the Demon.
In Golden Age DC comics, Priscilla Rich's jealousy and envy of Wonder Woman caused her to spawn a second personality. As the Cheetah, she became a recurrent foe in the Sensation Comics and Wonder Woman series.
In the 1986 remake of The Fly, Scientist Seth Brundle slowly transforms into a pathetic and diseased human-fly hybrid after accidentally fusing with said-insect in a teleporting experiment. At first, the deformities were purely physical, as his human flesh warped and decayed into a "cocoon" under which the monster grew. Sadly, he also transformed on a mental level into an instinct based animal, unbound by compassion and morality. The love of Veronica, our heroine, was sadly not enough to stop his predictions from coming true...
Averted in the sequel with their son Martin. The murderous rampage against his oppressors is all him. They are bad enough that this isn't a Moral Event Horizon for Martin.
Similar to Hide and Seek where Charlie completely takes over David.
Fight Club. Claimed by Tyler Durden to be the evil plan all along.
The VERY heavily fictionalised film of "The Boston Strangler" implies something like this at the end. The killer, Albert De Salvo, has been diagnosed with Multiple Personality Disorder (the real life De Salvo was never even suspected of having this condition,) and the film ends with him apparently having gone permanently catatonic after re-enacting one of the murders under the influence of his murderous side, and with no sign of the gentle family man left.
This happened in an episode of Single Female Lawyer, in which a woman had two personalities. She then received a court order to undergo a medicinal treatment that would cause her personality to stabilize (Against her wishes). Though, the medication overwrote the original personality, not the stronger more assertive one that developed later.
This happened to Dr. Jekyll at the end of his strange case. Luckily for Victorian England, Jekyll saw this coming and locked himself up before changing completely.
He locked himself up because by this time, all of London was out for Hyde's head, after all the atrocities he had commited. Hyde never tries to escape after this point; he just weeps in a corner, because both he and Jekyll know the end is near.
Happens to some of the Clayborn in David Wingrove's Chung Kuo series.
In the Skulduggery Pleasant books, Anton Shudder/his gist, Valkyrie/Darquesse and Skulduggery/Lord Vile. All of them double as Superpowered Evil Side.
In a rare case of the good side taking over (with a little help), from Gene Wolfe's story "The Death of Doctor Island", the psychotic teenager William, whose brain has been surgically divided between the left and right hemispheres, is deliberately subjected to a devastating trauma which leaves him catatonic. This allows his previously helpless right-brain personality, the calm and placid "Kenneth", to take control of his body.
Happens to Arnie Cunningham in Christine.
Happens in Artemis Fowl: The Lost Colony, when after Artemis and Co. escape from the Time Crash that's about to happen on Hybras, Qweffor (assisted by N°1) does it to the Big Bad Abbot.
And again in The Atlantis Complex. Artemis, having the titular disease, is taken over by his alter, Orion, multiple times.
Happens to one of the bad guys in Shade's Children: When the titular Knight TemplarVirtual Ghost uploads himself into a supercomputer, his conscience becomes personified in Robert Ingman, who is, conciendentally, Shade's original identity. As with the trope, Robert eventually destroys his dark side and helps the protagonists banish the local Dimension Lords. Too bad the other rule about redemption is played quite straight.
In The Lord of the Rings, the character Gollum's darker side of his personality ultimately ends up taking control over his kinder side.
Came this close to happening to Tahiri/Riina from the Star Wars: New Jedi Order series. Riina was not "evil", per se- though extremely messed up, she had a Pet the Dog in one of the few times she actually did manage to seize control of their body, and the merge helped Tahiri regain her sanity largely because each compensated for the other's issues (Riina's acceptence of pain and death helped Tahiri get past her emotional fragility following her boyfriend's death, while Tahiri's moral center held back Riina's more violent urges).
In The Pilo Family Circus, this happens to every single clown employed by the circus, due to prolonged use of the magical facepaint they wear: it takes around two years of usage for the new personality to completely merge with the clown's brain, but since the facepaint also grants the user superhuman strength, a newly-recruited clown has to wear it just to survive the early months at the circus, and by then, the clown's addicted. Jamie, the novel's protagonist, only just manages to avoid this fate.
This happened to a teenager named Saxon Hyde at the beginning of Michael Slade's Ghoul. That's especially unfortunate, because Saxon was the only one of this young man's personalities that wasn't a serial killer.
The first half of the Dragonlance: The New Adventures series consists of an evil wizard trying to engineer one of these. He implanted the preserved spirit of a powerful evil sorceress in one of his mindwiped servants to try to access her magical knowledge once she took over the servant. Needless to say, the sorceress is rather pissed when she does take over.
In Zen And The Art Of Motorcycle Maintenance the Narrator first tells us that Phaedrus has been killed by several thousand volts to the head. However, throughout the Chautauquas, it becomes clear that Phaedrus is alive and kicking and trying to gain control. Slightly different from the others, as it's the narrator's personality, the sane personality, that is the real takeover.
In Stephen King's short story Secret Window, Secret Garden, Mort Rainey's alter ego John Shooter claims Rainey "took the coward's way out". He briefly reverts to Rainey, but only after being shot.
Played with in Farscape. Thanks to Scorpius' neuro chip, Scorpius' "neural clone" gets stronger and stronger within Crichton over the course of the second season... until taking over completely in the finale. Even after surgery removes the chip itself, the neural clone, "Harvey", never really disappears.
Once the chip is removed though Harvey loses any power he had and is basically trapped in John's head forever, with John able to shut him up whenever he wishes. The best Harvey can do to John is be mildly irritating - unless his life is in danger, in which case, Harvey can muster up just enough power to be dangerous.
In the Peacekeeper Wars mini-series Harvey finally dies for good when the wormhole knowledge is deleted from John's brain.
We've heard that one before.
Basically what happened to Talia in Babylon 5. There was also a rogue telepath in one episode that goes through this.
Born vampires from Young Dracula all go through an attempt at this when get their full vampire status. How strong-willed they are seems to determine how much of their original personality is left afterward.
Implied to have happened at the end of a Married... with Children episode in which Bud's "cool" personality took over. It had absolutely no visible impact on future episodes (though perhaps that's the joke; Bud's so uncool that even his cool side is uncool).
In an episode of Criminal Minds, a young man's murderous and until-then unknown alter-ego permanently takes over his body, in an attempt to protect him.
An episode of Stargate SG-1 had Daniel controlled by several different personalities that got in his head.
Stargate Atlantis has a similar episode where Rodney and a (female) Marine wind up sharing his body. They each fight over control of the body, until it can't take their fighting anymore: if one of them doesn't give up and willingly disappear, both will die.
United States Of Tara: Alice, the "housewife" personality, reveals that she's jockeying to be the emergent personality if Tara ever reassumes therapy. This was followed by Bryce, Tara's alter based on her childhood abuser, who reveals that he wants to "kill" Tara.
Jekyll: When Klein & Utterson finally manage to capture Dr. Jackman, they put him into a device that deletes one of the two personalities. Unfortunately for Jackman's wife, Hyde is the surviving personality. However, when Mrs Jackman and her children are captured due to Hyde's immaturity, he resurrects his dead alter-ego and merges with him, mixing Hyde's incredible strength with Jackman's maturity.
After Spike regains his soul, he intermittently becomes evil again after The First Evil prompts him with a Victorian folksong.
In season 8, it's shown Oz came very close to going down this road willingly. He had been trying to deal with his werewolf side for a relatively long time, only to learn that whatever he tried was pointless and whatever ground he had gained before was lost. Eventually giving in and simply being done with it all started to look very tempting. It's never explicitly stated why he didn't go through with it, but his words "But I didn't" are coupled with an image of Bayarmaa, his future wife and mother of his son.
In The Sarah Connor Chronicles, Cameron experienced a case of this when her processors glitched due to combat damage, and Cameron slipped into an alternate personality mode that thought it was a woman named Allison Young, who was a resistance fighter in the future who was connected to the future John Connor, whom Cameron interrogated, mimicked the appearance and personality of, and subsequently killed.
A positive example occurs on The Twilight Zone. In the episode Nervous Man In A Four Dollar Room, a small-time thug (Joe Mantell) is ordered to kill someone for the first time in his life. His long buried conscience refuses to go along with committing murder and takes over as the dominant personality.
In one episode of Tales from the Crypt a lonely man's only companion is his own split personality. When the man starts dating a therapist, said split personality is afraid that she will destroy him and tries to goad the man into killing her. When the guy refuses, his other half decides to "kill" him and take over his life. In the end, the more assertive personality has cowed his former boss and is on the fast track to becoming an executive and is married to the therapist.
This is extremely common on Supernatural, due to the prevalence of demonic possessions and sympathetic monsters who want to retain their humanity but are incapable of controlling their predatory instincts. Memorable non-possession examples include Lenore and God-mode Castiel, after he ingests the Leviathan. Throw in manly tears, and you could almost turn this into a drinking game.
Warehouse 13: "The Ones You Love" reveals that the "Brother Adrian" that Artie's been dealing with all season is a hallucinatory manifestation of an increasingly powerful Enemy Within. Just as Artie realizes this, it grows powerful enough to assume control completely.
There's only room in here for one / And I've decided it's not you
I had to do what must be done / You would have done it too
BIONICLE: In one of the alternate universe Takanuva stumbles through, the alternate Makuta Teridax absorbs Matoro, who failed in his mission to save Mata Nui in this universe. When Makuta absorb other beings, they normally absorb their bodies and minds, but Matoro fights back, killing himself and Teridax, becoming the hero he was destined to be.
The video for Foo Fighters' "Breakout" has a meek Dave Grohl becoming the rock star Dave Grohl after losing both his medication and his shit (inspired by Me, Myself and Irene, on which the song was included).
In the world of Professional Wrestling, there's no other better example than Mick Foley himself. Along with wrestling as himself, he also wrestled under the personas known as "The Three Faces of Foley", which consisting of Cactus Jack, Mankind, and Dude Love. No one ever knew when he switch personas, resulting in one hell of a Wham Episode when he let Catcus Jack take over to fight Triple H in 2000.
The Harrowed from Deadlands are constantly in danger of this, since they owe their very existence to the demons possessing them. Except Stone.
In Twisted Metal: Head-On, Marcus Kane gives into Needles Kane, aka Sweet Tooth. Interestingly, however, it's not a complete takeover, but rather they seem to be working together, thereby making Dark Tooth and Tower Tooth (the two cars they drive together) the strongest in the game.
The bad ending of Soul Nomad & the World Eaters involve this, with Gig using your greed to destroy the main character's soul and taking over his/her body. The 'bad' ending of the Demon Path reverses the roles, with the main character destroying Gig instead.
Both the good and the evil ending of The Suffering: Ties That Bind results in either Torque or Blackmore wiping the other personality out completely.
The bad ending of Manhunt 2- achieved by killing people as gruesomely as you can - has Leo killing off Daniel's personality and becoming the sole resident of his body.
Near the end of Planescape: Torment you get trapped in a place where you encounter three of your previous "incarnations". One wants to take you over so and another simply wants to destroy you. If you manage to forcibly take over the other two or get them to merge with you, you will gain their memories (and a ton of XP). If you fail, you have to destroy them and their memories are lost forever. The third takes no convincing to get him to merge with you and is more than happy to.
Vampire: The Masquerade - Bloodlines has a scene where, after being led along by the two, you finally confront Jeanette and Therese Voermann — wait, make that Tourette Voermann. One half of her face is done up in Therese's professional businesswoman style, while the other half is done up in Jeanette's slutty schoolgirl style, and the two sides speak in turns about wanting to kill the other. Depending on the dialogue you choose, either Jeanette kills Therese, Therese kills Jeanette, or the two personalities decide to put aside their differences and coexist.
Done very strangely in the Warcraft series; Arthas was a paladin who is corrupted by the Lich King, becomes his champion, then later merges with him, but then Arthas takes over the Lich King, an entity of godlike mental power. A dark lord taken over by another dark lord who is later revealed to be slightly less evil than everyone thought him to be when they became one dark lord.
One of the main plot points of Riku's story in Kingdom Hearts: Chain of Memories and its remake is that Riku is fighting off the last of Ansem's darkness within his Heart. Several times he almost falls to it, and once he actually does, however how it happens depends on the version. In the original, after completely dominating Lexaeus in their fight, the latter blows himself up in an attempt to drag the latter down into the Darkness. However, in the nick of time, he is able to resist. In the remake, after a stalemate battle, Riku lunges at Lexaeus to finish him off, only to be backhanded into the ceiling and passing out. Unconscious Riku's body is possessed by Ansem who then kills Lexaeus in one hit before Mickey appears and frees Riku.
Fubuki Shiro, who is taken over by Atsuya, his inner mind's high ego little brother.
A positive example can be found in Deadly Premonition, where upon being traumatized and locked in the White Room by Forrest Kaysen, Francis Zach Morgan develops the personality of Francis York Morgan, who takes dominant control in order to protect Zach. At the endgame, Zach is given back control for the final confrontation with Kaysen.
In Mortal Kombat 9 a positive example appears in Ermac's Arcade Ladder ending. The soul of King Jerrod, Kitana's father and Sindel's husband, eventually becomes dominant in Ermac and becomes Edenia's protector again.
Super Robot Wars Alpha 3: Subverted When Cobray Gordon was formed from the fusion of Ayin and Ingram, Ingram tries to take over Ayin's body but ultimately realizes that he himself is being absorbed by Ayin
In The Elder Scrolls verse, the Daedric Prince of Order Jyggalag was cursed by the other Princes with this becoming the Prince of Madness Sheogorath. He can only briefly become himself again every few thousand years, and never long enough to permanently restore his realm. "Killing" and replacing him in Shivering Isles frees him to wander Oblivion again.
This is the major twist in the freeware Visual NovelOri Ochi Onoe. The protagonist used to be in love with the sweet and gentle Ori, but Ori turned out to be a split personality of the more violent and temperamental Ochi with the two personalities referring to each other as "sisters" and leading separate lives. Unfortunately, a later eye operation destroyed the Ori personality and left just the Ochi personality who the protagonist subsequently entered a very complicated and twisted relationship with. One of the game's endings has Ori regain control after the protagonist makes it clear that he loves only her, and another ending has the two personalities merge to create a more complete Ochi.
In Fallout New Vegas, Lily's aggressive alternate personality, Leo, completely takes over her if she goes off her meds, or you can convince her to take the recommended dose to completely suppress him. Similarly, Dog/God in Dead Money may have one of his personalities destroy the other, or merge the two.
He undergoes this before being sent into another dimension.
In an Aladdin episode, when a botched experiment leads to Mozenrath and Aladdin sharing the same body, Mozenrath tries to do this. He isn't successful.
X-Men Evolution, episode Sins of the Son. Charles Xavier discovers he has a son in Scotland, David Haller, and goes to meet him, only for two other boys to enter the fray as well. These three boys (David, Ian, and the evil Lucas) are all parts of the same being. Xavier tries to help expunge Lucas, but ends up locking away David, forever losing his son as Lucas goes off, never to be seen again (as the series was cancelled before this story could continue).
A curious example in Red vs. Blue: Reconstruction - when an AI is implanted in a character, the effect is described as being "of the same mind", where the two entities share thoughts. Though the host is usually able to maintain his or her identity (even if free will is lost), Agent Maine (better known as the Meta) degenerated into a mindless husk after playing host to 7 different AIs at once and stayed that way even when the AIs were removed. Of particular note is the fact that all 7 AIs were Literal Split Personalities of another AI, the Alpha.
Done in a masterfully nightmarish way in Marble Hornets. In Entry #61, we're treated to a video of the hooded masked man stealing Tim's medication while Tim is in the bathroom during a coughing fit. After Hoody hides, Tim stumbles into the room and undergoes a disturbingly realistic seizure, then jerkily climbs to his feet and absconds, all with the heavy implication that his other personality, the original masked man, is back in the driver's seat. Amongst the evidence that Masky is in control is his pronounced limp- after his leg was previously broken, he'd been limping up to his last chronological appearance in the series.
Homestuck: Calliope, a sweet and genuinely kind girl, shares a body with Caliborn, a barely restrained sociopath. At the end of Act 6 Act 3, Caliborn becomes not so restrained, escapes from his confines, and starts a session on his own, taking over their body completely and burying Calliope in the process. However, some characters believe that she may not be entirely dead and that in fact finding her might be the key to defeating Lord English once and for all. The author mocks Caliborn for doing this, since he stunted his own Character Development by averting the Split Personality Merge that Cherubs normally go through.
An episode of the dramatized court-case anthology series The Judge dealt with a female dissociative identity disorder patient who, in the episode's surprise twist, turned out to be the victim of her psychiatrist, who was having an affair with her Femme Fatale persona and plotting to destroy her original personality so the two of them could be together. While the script might've involved some dramatic exaggeration, there are real-life cases of secondary personalities in DID cases plotting to kill their primary selves, without recognizing that if they're successful, they'll die too.