"But I'm not my "kind", not anymore — your shadow leeches saw to that. I'm half Toa of Light, half Toa of Shadow. This was what you wanted, wasn't it? Toa consumed by darkness, Toa not hampered by things like mercy or morality. Well, now you can choke on it."
In Magical Project S Misao is normally a shy, physically weak and powerless girl, but when she transforms into her evil side, Pixy Misa, she becomes super confident and super competent. At full power she is able to easily defeat the heroine Sasami.
Himura Kenshin from Rurouni Kenshin is a Technical Pacifist in battle, fights with a reverse-blade sword and vowed years ago to never kill again. But if things seem to be getting out of hand even for him to handle (usually when his friends are in danger), his fighting style and mindset slowly begin changing into the highly pragmatic and ruthless Battousai, the assassin he was ten years ago. He switches the extremely humble sessha for the brash ore, loses the de gozaru from his speech, his eyes turn gold and more slanted and he moves for the kill. In the anime, Battousai seems to be considered an actual personality Kenshin has learned to seal off by creating his rurouni persona - he has to punch himself in the face to push it back and revert back. In the manga, this gets lampshaded a bit when Kenshin remarks that his face muscles have cramped and he isn't able to end his permanent death glare.
It's an unusual case, as in his current state Kenshin is actually much stronger than when he was the Battousai, having fully mastered Hiten Mitsurugi Ryu, including learning its ultimate technique, in the intervening years. But as the Battousai, he never held anything back, always going for the immediate kill. If he didn't stop himself every time the Battousai persona tried to emerge, though, he'd combine his increased strength with his former ruthlessness to become a near-unstoppable killing machine.
Son Goku of Dragon Ball faced this early on with his Oozaru transformations. In this form he loses all sense of self while gaining immense strength and size; the best solutions found were to either cut off his tail or blow up the moon. The transformation into the Oozaru, while generally a bad thing, did have two positive outcomes. One was the escape of Goku and friends from Emperor Pilaf; the second was Gohan defeating Vegeta... by landing on him.
The Super Saiyan state acts like this without proper training. The transformation occurs in a moment of pure rage, and the resulting personality shift leaves the Saiyan struggling with an overwhelming anger.
Frieza even comments on this when Goku beats him around as a Super Saiyan. He asks if Goku isn't being as cold blooded as he is, and concludes that Goku is still struggling to control the rage in him and that it's uncertain if he will win.
Son Gohan during the Cell Games is a chilling example in anime. It is more than a little disquieting to watch the kind-hearted and sensitive little boy we have come to care about over three sagas turn into a gleefully cold-blooded sadist when he achieves the Messianic power of Super Saiyan 2.
Later, Vegeta undergoes an evil transformation when he lets Babidi control his mind. Babidi's mind control does make the user stronger, as it is first witnessed with normal human like Spopovich and Yamu, who become more powerful. The increase of power from Babidi's mind control is why Vegeta let's Babidi use his spell on him because he wants to be stronger when he fights Goku.
Played Straight in Bleach with Hollow Ichigo, a monochromatic version of Ichigo who is more aggressive and fights with a bizarre fighting style as compared to Ichigo's. Justified that he normally pops out whenever Ichigo's life is threatened because if Ichigo dies, his Hollow self dies as well. Hollow Ichigo mocks Ichigo's weakness at every opportunity, his sword energy is jet black, and his Stepford Smilerfacial expression deeply contrasts constantly-frowning Ichigo.
Ichigo gains some control over "Hollow Ichigo" while training to become a Vizard, enabling him to voluntarily call forth a full mask and consequent powers. However, at the start, he can only maintain this form for 11 seconds. By the time the Huecho Mundo arc rolls around, the mask has become just another power-up, its 'evil' being utterly gone and leaving Ichigo in control. This wasn't achieved without a climactic Battle in the Center of the Mind, however. Twice, even.
Kubo is in love with this trope. Ichigo's Hollow Form has several different states, including one that's purely homicidal to the point of even attacking allies if they get its attention. Recently, it's gone back to square one now that Ichigo's regained his Shinigami powers. Nobody knows who, where, or "what" Hollow Ichigo is at the moment.
It's also important to note that in the Bleach universe, Dark Is Not Evil and Light Is Not Good when it comes to color schemes, with white representing death, and Hollow Ichigo is almost completely white. In fact, Kubo was so dead set on this color symbolism that the original title was supposed to be Black, but then he decided that Black was too unoriginal, so he changed it to White. White had the same problem, so he eventually settled on Bleach.
Hollow Ichigo being Ichigo's true Zanpakuto spirit complicates things even further. Beneath all that aggression, Hollow Ichigo only ever wanted to protect Ichigo.
Subverted in that eventually Tessaiga itself is so powerful that his demon side is actually weaker by comparison. (He was in fact given so powerful a weapon so he wouldn't need to rely on his own demon powers.)
Suikotsu of the Band of Seven is a doctor with Split Personality whose darker side is a sadistic, blood-thirsty murderer. It turns out that this is his true self, as well: he'd been good, gotten a taste for battle when defending his village, and became a mad killer of his own free will. The purifying effect of the mountain the story takes place on lets him revert to the decent man he'd once been.
Subverted in Naruto where Jinchuriki can attain great powers due to being linked to their Tailed Beasts. However, only Naruto and Gaara are explicitly shown using this trope due to their Dark and Troubled Past, manifesting when either are emotionally distressed or when their lives are at stake. Both manage to reject or tame these circumstances, the latter turns his into a Super Mode.
Sasuke and the curse mark. During the Chunin Exams arc, Orochimaru gave him a curse mark, which, once activated, turned the quiet, elite loner into a sadist who broke a guy's arms without a second thought. Creepy! "You seem quite attached to these arms of yours."
In Hellsing, Seras Victoria's berserker mode could be counted as her Superpowered Evil Side.
Lucy from Elfen Lied is a variant of this trope - while she was originally the 'true' personality, she suffered a nasty head wound that caused Split Personality and left her nonpowered good side, Cute Mute Nyuu, in control. The times when her 'true' self resurfaces fit this trope to a tee.
In the manga, we learn that Lucy - or rather, Kaede - is actually more of a superpowered neutral side, who's been listening to and partly controlled by a completely insane Omnicidal Maniacthird personality (created by either her Diclonius instincts or by snapping as a result of thetormentinflictedonher when she was young, whichever theory you prefer) that wishes to annihilate the entire human species, if not all sentient life.
Orson from Record of Lodoss War is a Berserker Class and, when he gets angry, with the influence of the demon Hyuri he becomes an unstoppable killing machine that wields a massive claymore as if it were a toothpick.
Vash the Stampede's eponymous weapon in Trigun is triggered involuntarily by such events.
There is less 'evil' here than 'Evil Twin pulled the trigger' or occasionally 'moment of Unstoppable Rage.' Vash has never shown a superpowered evil side, although he does have the related tropes of Crouching Moron Hidden Bad Ass, an Evil Twin, and the occasional burst of aforesaid temper, after he witnesses a massacre or something else heinous.
Yugi's powerful alter ego in Yu-Gi-Oh!, Yami Yugi, is not actually evil, but he is a lot less concerned for the well-being of his enemies. Yami Bakura and Yami Marik, on the other hand, are both gleefully, psychotically evil. (Regular Marik isn't exactly a nice guy either.) In the manga, however, Yami IS evil, at least initially. Word of God stated in a Shonen Jump Q&A that Yami Yugi actually WAS evil at first, due to a few millennia of his soul being imprisoned, but that his experiences with Yugi and his friends gradually helped him regain his sense of righteousness. (Quite likely, the turning point occured during the first encounter with Shadi.) The plot point was revisited in the second anime in the "Waking the Dragons" arc. After Yugi's soul gets imprisoned, leaving Yami in control 24/7, he starts to regress back to his original evil nature, without Yugi's kind nature there to temper his drive for vengeance. Which makes him a more vulnerable target to Dartz's machinations.
Fubuki is possibly unique among this trope in that he has, twice, purposely assumed it, and had a rather good reason both times (well, good in his view, maybe). The first time he thought it might bring Kaiser back to his senses, seeing as he thought Kaiser might have been under a similar curse. (It didn't work, seeing as that was not the case.) The second time was because he felt that bringing Darkness back was the only way to answer the questions tormenting him.
The eponymous warriors of Claymore constantly run the risk of becoming "Awakened Ones" when using their powers. In that state, they become their superpowered evil sides without any chance of turning back to their original forms.
The twist is Heroic Willpower lets the sufficiently motivated come back from the brink and retain some control after completely transforming.
Quiet and retiring Haru's alter ego, "Black Haru," from Fruits Basket is a humorous take on the trope. In the anime (or early in the manga), at least. It's played for some serious drama later in the manga.
Along the same vein, that goes for the brief 'cameo' of "Black Yuki" in the later chapters as well.
When Yuuri from Kyo Kara Maoh reaches an appropriate level of moral outrage, he involuntarily transforms into a larger version of himself with cooler hair, amazing magical powers and a thirst for justice.
...who almost always says he's going to kill people, even though he 'dislikes bloodshed', but has no confirmed kills, and is usually confirmed as not having hurt anyone. Once he sentenced an Unlucky Childhood Friend and her quarry to go on a date. Another time he mistook an assassination attempt for an earthquake and saved everybody, even the enemy soldiers who'd been trying to kill him. Also, he has been known to say nonsensical things about fire brigades etc., just like Yuuri always does, just in a serious, booming way.
In the manga he gets a lot closer to the 'evil' line, but is a lot less divorced, in terms of identity, from normal Yuuri, whose lifelong tendency to a really violent temper is underlined. But the manga is darker than the anime. And Yuuri accidentally stabbed Wolfram half to death a while back in the novels. Genre Woobiefication?
When Kazuma of Kaze no Stigma taps into his evil side (really a reversion to his old behavior in his backstory), he will do whatever it takes to accomplish his goals, no matter who he has to hurt (or mind crush!). Kazuma may actually be a subversion if Ayano is correct in her analysis when she faces him to try and stop him from going on his rampage. She states quite clearly that he used to be so overwhelming that it seemed to be stupid to be afraid of him. Because she was afraid, it was proof that he was weaker than before. In any case, he's still very kick ass.
She's right. When he's in this state he's basically letting his anger and pride control him (something he had often warned others against), lacking the finite control that made him so powerful in the first place and preventing him from taping into his full powers. Given that this fight is also the first time in the series that we've seen her overpower Kazuma both Verbally and Physically...
In JoJo's Bizarre Adventure part 5, the protagonist Giorno Giovanna develops a new stand "Golden Experience Requiem" which is intelligent and can act on its own, and completely destroys his enemy Diavolo, causing him to experience death over and over again for all eternity.
Ginji's Raitei persona in Get Backers. Kazuki also has one, when he releases the seal on his Stigma eye. The same way Ginji is the "Thunder Emperor," he's "the Prince of Terror."
In Daa! Daa! Daa!, you can see a crystal-clear example in Christine, the older Hanakomachi... when a girl (human, alien, it doesn't matter) approaches her beloved Kanata in a remotely romantic way, she emits a black aura and her eyes glow: it's your cue to get away. In such a state she is capable of feats like lifting trees & breaking concrete walls. She's not evil, just clingy.
Moka's "super-vampire" side in Rosario + Vampire. Super-Vampire Moka speaks to her normal self through her Power Limiter, giving her guidance that she needs. This one isn't evil so much as very emotionally cold, though.
Though recent chapters have revealed that Inner Moka is the true personality of the one body and the Outer Moka was created when Moka's powers were sealed by her mother's Rosario so she could go in to hiding. Outer Moka's personality is actually based on her mother Akasha's personality.
Tsukune's ghoul transformation is much closer to the trope. He requires a power limiter like Moka to keep it under control, but unlike Moka, he's a complete monster without it.
As of Chapter 41 it seems he's at great risk of becoming a second Alucard.
In Heroic Age, the giant mech-like warriors of the Heroic Tribe, called the Nodos, can eventually let out a Superpowered Evil Side when two fight together - whether against each other, or side by side - called Mental Chaos, when they enter a pure blind berserker rage. Considering a single Nodos can sometimes destroy a whole planet on its own at its normal levels of power, a Nodos in Mental Chaos is a frightening thing indeed. In Bellcross, the Nodos that serves the human Age, Mental Chaos is shown by his skin and "armor" turning dark gray, almost black, and a set of blue spheres implanted in his body begins to glow bright red. And he roars a lot.
Guts, the protagonist of Berserk, is known to slip into this state during particularly tense moments.
In the manga, this becomes personified by The Beast, a feral lupine creature formed from the considerable amount of rage at the Godhand (and Griffith in particular) and other negative emotions that Guts suffers from after the Eclipse. When Guts is under The Beast's influence, he becomes a danger to everyone around him, even the people he loves — at one point in the manga, he almost rapes Casca while under its influence. Later on, he acquires the Berserker Armor, an extremely dangerous cursed Dwarven artifact that blinds its user to pain and limitations and causes them to see everything as a threat. It also becomes a kind of physical conduit for The Beast.
In the h-anime Makai Tenshi Jibril (Translated title: Jiburiru - The Devil Angel), Meganekko-with-a-crush Meimi is given the sadistic Magical Girl persona of Misty May, the "Devil" counterpart to the heroine Rika (St. Jibril/Jiburiru).
Closer to the trope definition would be Rika's evil form in the sequel, or Hikari's Black Aries form in the third game.
This happened in Guyver once, but it was a doozy. After his father was turned into an Enzyme Zoanoid and ripped out Sho's brain, the Guyver itself took over Sho and killed his father. Let's just say that when he found out, he didn't take it well.
Though this wasn't so much the Guyver being evil, as obeying its Prime Directive of wearer-preservation.
In D.Gray-Man, Allen's Innocence seems to have a mind of its own and tends to upgrade itself when Allen gets particularly angry or emotional. It also can control Allen when he's unconscious, and it tends to be very vicious. Then again, it's only dangerous to Akuma.
Also Arystar Krory, although he has less of a Superpowered Evil Side than a Superpowered Deadpan SnarkerBad Ass Side.
Well, Tyki Mikk comes in black and white, but I don't think Noah consciousnesses change, except in that one stabby incident. 'Tyki Mikk, now in black, white, and Roiling Chaos. White out of stock.'
Allen, with the Fourteenth Noah who is now making himself heard in conversation, if only to mess with the Millennium Earl's head. The Fourteenth hasn't been seen in actual combat yet, so 'superpowered' may still in question (yeah, right), but his brief awakening in Allen is enough to terrify a Level 4 akuma and his ability to make Allen display a truly nasty Slasher Smile definitely hints at the evil part.
In the most recent chapters, after Kanda stabs Allen and awakens his Noah side, there are clear hints that the Fourteenth is terrifyingly powerful, even by Noah standards. Remember, this is the guy who apparently by himself managed to kill off nearly all the previous incarnations of the Noahs and severely injure the Millennium Earl several decades ago.
Son Goku from Saiyuki. His 'evil side', the Seiten Taisei, is akin to a heretical chaos god and nearly overthrew the Heavens, goes on an Unstoppable Rage whenever unleashed, and usually ends up trashing both the enemy, AND his teammates by the time he's finally stopped. The only thing that keeps him in check is the crown he wears which serves as a Restraining Bolt.
Cho Hakkai's youkai form would normally just be a Super Mode, but when he changes into it he risks slipping into this due the Minus Wave that is sending youkai berserk. (And consequently, he doesn't pull it out unless the situation is beyond dire and it's time to set his usual nice-guy facade aside, meaning that even without the Minus Wave there's going to be blood on the walls when he's finished.)
Soul Eater is infected by some of Medusa's black blood, which manifests itself as a devil living inside his head offering him power in return for giving in to the insanity. On the one occasion he and Maka give into it in order to stand a chance against The Dragon, the trope is almost fulfilled to a tee... Except it's not so much a 'superpowered evil side' as a 'Nigh Invulnerable giggling lunatic side who catches The Dragon completely off-guard'. They barely make it back to sanity, which the devil seems to consider this a temporary setback.
While not anywhere as drastic a change as the above, Stein's tactic against Medusa, of giving into his own madness, could also be seen as this. Though this side is not so much 'hidden' as it is what Stein would be if he didn't hold himself back through fear. He also seems able to control it frighteningly well, if Spirit's comments at the time are anything to go by.
Lina from Slayers when she's possessed by the being that accidentally created the universe and was presumed to be ready to undo it just as willingly. However, it is instantly subverted as it chastises the villain that made Lina fall to it and unmakes only him and all his plans. Her actual motivations are stated to be simply alien to our moral scale.
The "berserker spirits" aka The Dragons from Ikki Tousen. So far, we have four dragon holders: Hakufu, Ryuubi, Sousou, and Ryoumou
Sonic X: Absorbing the negative energy of a factory's worth of fake Chaos Emeralds coupled with seeing his friends tortured unleashed Dark Sonic for the first time in animated continuity. Complete with white eyes and navy-dark energy all over his body, plus the ability to tear anything that pisses him off to shreds.
The main character of 666 Satan is actually the container of Satan himself along with nine other demons that have the ability to possess people. There are also ten angels, none of which are evil or trying to take over but some are incredibly vain and judgmental.
While not necessarily "evil," a mechanical version of this can be found within Gundam Wing, specifically in the form of the Zero System, a computer system in the Wing Zero which interfaces with the pilot's brain, enhancing his reaction time and aggressiveness, and allowing him to process tactical data with superhuman speed and efficiency... with a fun little side effect of driving the pilot temporarily Ax-Crazy. Given that they're still inside a giant, city-killing death machine with all their piloting skills boosted, results are predictably messy.
The Zero System can be used safely: the pilot must keep a clear head and not be conflicted in his thoughts towards the battle. In this case the pilot then gains all the benefits without the insanity. Both Heero and Zechs reach this point in the end - and, in fact, Heero eventually becomes so relaxed about the obligatory in-cockpit Mind Rape that he starts offering it to his fellow pilots as therapy. Astonishingly, it works.
Gundam 00 also has fun with this - Allelujah Haptism has a Split Personality: Hallelujiah, a blood-thirsty "warrior" half who is more skilled than any other pilot in Celestial Being.
Then subverted with The "Ultimate Soldier", aka H/Allelujah or Doublelujah, whose an even better pilot and integrates the best parts of both personalities, including Allelujah's morals.
Then there's the Unicorn Gundam. Normally a high performance mobile suit with generally above average statistics but nothing of any particularly remarkable quality, when the pilot and it come into contact with a Newtype enemy and the pilot feels a strong enough emotional pull (usually anger), the Gundam feeds off of it and transforms into the eponymous Unicorn Gundam, now in its "NT-D" (Newtype Destroyer) Mode. Its speed ramps up significantly, its strength seems to increase, it gains new abilities related to its psycoframe, and worst of all the suit's AI will stop at nothing short of a manual override or a Heroic RROD to kill any Newtype in weapon range. This is entirely not good for anyone who happens to be facing the thing down, as it generally goes from on an even footing or slightly underperforming the enemy to stomping their face in on a one-sided epic beating.
After falling victim to this once or twice, Banagher learns how to consciously activate the NT-D system and override its AI, leaving him in complete control of the suit.
In Gundam SEED, Andrew Waltfeld seemed to likened SEED Mode to berserker rages of the Vikings. While it's not an actual Superpowered Evil Side, activating SEED Mode while enraged is certainly a glorious mess.
Dark Schneider/Lucien in Bastard!!. Tiny, fifteen year old boy turns into the infamous eponymous bastard, the all-powerful Exploding Wizard. Then subverted since Darsh does a lot of dorky things outside of battle and seems much more like a Jerk with a Heart of Gold than anything else, with some people often remarking that compared to the Dark Schneider of fifteen years ago, this one is practically a saint.
In Saikano, Chise's personality while in the Ultimate Weapon form could be psychotically vindictive, threatening on multiple occasions to utterly eradicate anyone who got in her way or made her mad, be it friend or foe... and that she could do so with ridiculous ease made her even more horrifying.
Parodied in YuYu Hakusho. Yusuke dies, and it is revealed that he is in fact part demon. Said demon blood kicks in and he comes Back from the Dead, with his demonic powers lending him a considerable increase in strength. He then decides to have a little fun at the expense of some alarmist commandos who wanted to take him out.
Yusuke: For what I desire is ending the world and everyone in it. For I am the mighty Mazoku, Lord of Evil! And now, after hibernating for a thousand years I will finally reveal my true form! AHAHAHAHAHAHAHAHAHA... psych!! Damn, you guys are gullible!
Late in his fight with Sensui, the trope is played straight, as while he still isn't quite able to overpower him, his demonic ancestor Raizen takes control of his body and uses it 'better,' defeating Sensui easily and killing him, which Yusuke is not cool with. Mostly because he wanted to kill Sensui himself...
Sensui himself is an inversion of this trope. Of his seven personalities, the strongest one is Shinobu, the original one, and the only one to remain pure and innocent. His Kazuya personality, by far the most violent and sadistic, is the weakest personality shown.
Actually Kazuya is the second strongest, so the trope is played straight if you compare him to Minoru (Sensui's manipulative personality) but inverted compared to Shinobu.
Although the above analysis falls apart if you consider Shinobu's "innocence" an Informed Attribute.
Kurama displays a variant on this trope in the Dark Tournament when his opponent tries to reverse his aging to make him a baby, but goes too far and reverts him to his previous incarnation as a powerful demon bandit.
Mayo Kagura from Mai-HiME Destiny. In a series where everyone has psychic powers, it's hard to stand out when you have boring ol' telekinesis... unless you have a Superpowered Evil Side that can uproot trees and use them as weapons.
Ryo Takatsuki from Project ARMS. His right arm is a nanite-based entity known as the Jabberwock. Get him ticked off enough, and the Jabberwock spreads over and takes control of his entire body. Though more really, really angry rather than outright evil, this doesn't stop Jabberwock from doing such feats as tearing apart a helicopter division, numerous other ARMS, emitting fire, and much, much more. Oh yeah, and it evolves too.
In Omamori Himari, all demons, including Himari, can fall prey to their instincts or desires and become savage beasts. In Himari's case, she turns into a nigh unstoppable juggernaut-but, like any other demon, she runs the risk of not being able to return to normal. It's explicitly stated that in this situation, they will be exterminated.
Played with in that when he learns that the superpowered evil side is a product of the Magia Erebea turning him into a demon and that there's no known way to stop it, rather than decide he'll figure out a way to seal it away permanently, he instantly, without hesitation, decides to get the whole demonic conversion process over with right now and get it out of the way.
Technically he agreed without fully understanding how it worked, and Eva just kickstarted the process. Given that the alternative is well, the same thing only over a longer time period, and that the point of the conversion is to get the Superpowered Evil Side under control, it was really the only viable option anyway.
Pikachu in the Pokémon anime practically turned into a demon when he was possessed by Groudon.
In Air Gear one of the strongest members of the protagonist's team has a split personality. Although Agito isn't exactly evil, his aggressive, bloodthirsty behaviour and great skill at the series' title sport make him this to Akito, one of the gentlest characters in the series. Additionally, later on Agito/Akito discovers he has a third personality named Lind who embodies this trope. He's the polar opposite of Akito and is implied to be much better at Air Gear than Agito. Although his intentions are still largely unknown, Agito himself warns the protagonist that Lind will probably end up becoming the main personality and betraying him.
Change 123 demonstrates why, in the real world, this is less of a cool new power and more of a deadly serious problem; you don't want your high-school girlfriend to snap and try to kill someone on your behalf because they said nasty things about you...or to try to kill you because you tried to stop her from killing that person.
The anime movie Tekkon Kinkreet has the main character Black get saved from space aliens by his own Superpowered Evil Side known as "The Minotaur" which then tries to completely overtake him and show him the true power "The Minotaur" can unleash.
Not really berserk but the dummy plugs do battles the messy way. Just look at Unit 01's Rei-plug tearing Unit 03/Bardiel to pieces or the MP Evas' Kaworu-plugs gutting Asuka in End of Evangelion.
Unit 01 again during the Zeruel battle. It behaves more like a rabid gorilla than a human. Once it eats Zeruel's S2 Engine, it just stands there and roars to the sky.
The Rebuild of Evangelion 2.0 version of that battle turned out to be a Crowning Moment Of Awesome as Unit 01 Mind Raped Zeruel to death while Shinji was in control the whole time. Keeping with the heavy symbolism of NGE, Unit 01 even manifested a halo about halfway through the Curbstomp Battle. And after absorbing Unit-00's core and Zeruel's body into itself, it became so powerful that it began to unmake the world just by existing. Good thing Kaworu's Eva has a very good throwing arm...
Mari hijacks Unit 02 during the Rebuild 2.0 Zeruel battle and, when she fails to gain the upper hand, triggers Beast Mode. Even if an Eva goes berserk, it has restraints. Now you see what happens if the pilot willingly removes those: it behaves like a rabid xenomorph, snarling, salivating and making impossibly cool leaps.
In Violinist of Hameln, Hamel's mazoku blood gives him a Superpowered Evil Side that, however, is not usually activated by rage or being near death, as it usually the case, but instead takes over whenever he cannot carry on anymore and is close to Heroic BSOD. It comes with demonic physical changes, such as horns and wings, is powerful enough to easily devastate whole cities, indiscriminately murderous and completely uncontrollable until the final battle, when Hamel managed to stay in control despite the full-body mazoku transformation, because that time he fought for his friends and love and only transformed because the Big Bad's power level was high enough to warrant it.
Tohma in Magical Record Lyrical Nanoha Force has his Black Knight form. When it comes out, he gains a Healing Factor, powerful magic, and is capable of performing a Zero Effect, which greatly drains the energies of anyone and anything in a wide area. Unfortunately, he loses over half his consciousness in this form, replacing it with an insatiable need to kill.
In DNAngel, Satoshi has Krad, one half of an extremely powerful artwork who likes killing people, living in his head. Daisuke has Dark , the other half of the same artwork living in HIS head, although Dark's more mischievous than evil.
I'd like to enter into the record Krad's fandom nickname: Homicidal Blonde.
Brain from Fairy Tail has Zero, who emerges when the Oracion Seis are defeated.
Also Rogue, whose magic causes the darkness in his heart to manifest as his shadow, tempting him to do evil and even possessing him once to give him a power up and try to force him to kill.
Hotaru Tomoe from Sailor Moon has two other sides. Sailor Saturn and Mistress 9, who nearly destroyed the world.
Hotaru also subverts this trope. Sailor Saturn's breed of destruction is basically a cosmic Reset Button - albeit one that's only marginally better than outright annihilation. Mistress 9, on the other hand, is desperately weak; the Deathbusters' scheme in the second half of the season boiled down to saving her before she (and Hotaru) curled up and died.
Also Chibi-usa, who had one as well when she was brainwashed by the Wiseman, known as the Black Lady.
When Yaiba loses control of the Raijin Sword he turns into a lightning-blasting, destructive avatar of the God of Thunder.
In the series A Certain Magical Index, the main character Touma was shown to have an actual sentient being inside of him that appeared after having his arm cut off. The guy who cut off said arm had an Oh Crap moment when he realised that his power paled in comparison even though he reached the point of being higher than God. (This is a given because Touma's last name Kamijou can be translated several ways, one being "Invisible Demon".)
One Piece has Chopper, a shapeshifter who normally has three forms available. When he eats a Rumble Ball, he gets access to four more forms for three minutes. If he eats another before the cooldown wears off, he gets three more minutes, but the shapeshifting is uncontrollable. If he eats a third, he morphs into a giant monster that's incredibly powerful, incredibly resilient, and absolutely uncontrollable. In its first appearance, he one-shotted an enemy who had been beating him handily a moment before, then smashed through a castle wall and went on a rampage, destroying a dozen rooms before Franky managed to throw him into the ocean.
As with Chapter 645 Chopper has now mastered his monster form.
In the kingdom of Rommel, there was phenomenon where it seemed as if the wind itself was cutting people. Eventually, a Marine warship was sent out to investigate, and the culprit was found to be a man by the name of Hakuba. Even to this day, he still hasn't been caught, but he still appears in towns where a certain pirate resides. The pirate in question? Cavendish. In fact, Hakuba is the one who went out to sea to become the pirate named Cavendish.
Piss off the softspoken, easygoing and always smiling Wolfgang Grimmer, from Naoki Urasawa's Monster, and you'll have to deal with "The Magnificent Steiner". Carnage ensues - though it's never directly shown.
The main character of Ikki Tousen, Sonsaku Hakufu, is a token Idiot Hero with enough fighting skill to beat the crap out of a school attended only by martial artists and assorted thugs. But sometimes the enemy is just above her level and she finds herself beaten almost to death: vision blurs, strengh fades... Congratulations! You've awakened "the power of the dragon" and the last thing you are likely to see is her raptured expression, while she strangles you to death or is about to stab you with a sword. To be honest, everyone possessed by a dragon soul behaves this way... even if not a fighter.
Akira Fudou and Amon from Devilman may be the oldest example in the history of manga. And also an Unbuilt Trope, as the manga shows that having one wouldn't just conveniently let you send it out on enemy too strong for you to take - taking Amon's powers changes Akira from meek and gentle boy into complete Blood Knight and that even without letting Amon out. Amon however only breaks free in a Spin-Off manga AMON, and an OVA of the same title. In the anime Amon eats a little girl alive and in the manga he is maybe just a bitter Anti-Hero, but still destroys several countries, just by passing through them.
Kagura, the adorable, profane, sukonbu-chewing, nose-picking heroine from Gintama goes from this◊ to this when the self-imposed chains on her Yato blood get broken. She stops fighting to win and instead fights to kill with a savagery that scares her.
The protagonist of The Three Eyed One, Hosuke Sharaku, is a tiny, wimpy, Charlie Brown-esque boy who turns into an immensely powerful, larger, psychic supervillain when the bandage on his forehead is removed, revealing his third eye.
In Steel Angel Kurumi, at least the anime version, it's revealed that Kurumi's Angel Heart Mk. II has both an angel and a demon in it. Thanks to Nakahito commanding Kurumi not to hurt people, Kurumi finds herself helpless during fights because she doesn't want to hurt anyone, especially her fellow Steel Angel sisters. The demon side manifests soon after, Blank White Eyes, grim demeanor and the ability to punch holes into walls from a distance. When Nakahito's kidnapped in an attempt to find out why his power makes Kurumi stronger, she starts to freak out and slip farther and farther into this power until everyone is forced to use a last-ditch weapon to kill her. Thankfully, Nakahito's able to break through to her and True Love's Kiss ends up purifying her heart.
In Anpanman, Rollpanna is already a superheroine, but with two hearts: a good one and an evil one. Being under the influence of Baikinman or being near Anpanman triggers her evil heart, which powers her up even more, but she can snap out of it when by her younger siblings, Melonpanna and Creampanda. Sometimes, Baikinman uses dirty tricks to completely remove her good side, turning her into Black Rollpanna, a ruthless fighter that is even more powerful than when she has both hearts, and nothing stops her, not even her siblings, until both hearts are together. For these reasons, she prefers to wander the earth alone, in fear of her evil heart being triggered.
Vampire Hunter D: Dhampires (human/vampire hybrids) can 'vamp-out' under physical and/or emotional stress and start exhibit Noble-like traits such as drinking blood, etc. Given how powerful D is when he's trying to act mostly human, when his Noble side emerges, expect plenty of Oh Crap moments and Curbstomp Battles.
Kyoukai no Kanata: Akihito loses control of his youmu side after sustaining severe injuries. He is immensely powerful in this state, requiring 3 people just to contain him. One stray fireball creates an enormous explosion when it hits a nearby mountain and he levels his surrounding area.
In the Alan Moore Retelling of the comic Marvelman, Miracleman, Kid Miracleman is the only survivor of a nuclear blast (he thinks) and stays in his superpowered form for years. In this form, but using his civilian name of Johnny Bates, he grows up. His moral code begins to erode because he is the most powerful man on earth and he has no one to tell him what to do. When Miracleman confronts him years later, he is the CEO of a computer company and a rather nasty piece of work. They fight, and he is forced to change back into a normal human child (His other self didn't age for years while in stasis) and left at an orphanage. There he is constantly bullied by other orphans, having to carry the dreadful knowledge that he could become more powerful than any of them, but he dare not because KM would start killing people. Then he is pinned one day, and some of the boys attempt to gang rape him, and this pushes him past the breaking point. The resulting awakening of KM results in the slaughter of the boys and a significant number of the population of London. Miracleman is forced to make him transform back into a human and end his life with a Neck Snap.
Wolverine has a tendency to lapse into a "berserker rage" while in close combat. In this state he lashes out with the intensity and aggression of a mindless animal and is even more resistant to psionic attack. Though he loathes it, he acknowledges that it has saved his life countless times.
It's still not clear whether Dark Phoenix is Jean Grey's Superpowered Evil Side or a variation on Grand Theft Me. (Whether Jean actually became Phoenix (and then Dark Phoenix) or was merely replaced by the Phoenix Force is in an almost constant state of retcon, so the confusion is quite understandable.)
The New Mutant Magik, way back when, was partially transformed into a demon by Belasco. Her "Darkchylde" side is intelligent but malicious, though almost powerless on Earth, where her magic doesn't work very well. In the end it took over her and nearly ended the world in a major Crossover Event.
Legion from the same series was first introduced with a Superpowered Evil Side in Jack Wayne, as well as a Superpowered Jerkass Side (the pyrotic Cyndi). He usually has one or more evil personalities, except when they are temporarily merged.
His father Charles Xavier also sometimes suffers from this trope. First, in a crossover mini-series with the Micronauts there was the Entity, a personification of his dark side. Later there was the supremely powerful Onslaught, who also contained elements of Magneto's evil side. Then there is Cassandra Nova, who combines this trope with elements of the evil twin and alien invader.
Super Sonic in the British Sonic the Comic comic series. In that comic's canon, the Chaos Emeralds are full of pure evil (as energy), rather than their neutral alignment as confirmed as canon by Sonic Adventure. Super Sonic came about by overexposure to the Chaos Emeralds, and as such... he's insane, and evil, sadistic, psychopathic... and equipped with the power of the Chaos Emeralds. When he split from Sonic and lost his powers he mellowed out, but returned to form when he got them back. The Archie comics did a homage to this in one issue, although in that canon there's generally no personality difference between Sonic and Super Sonic, Doctor Robotnik is also this to his former self Dr Ovi Kintobor, due to exposure to raw evil Chaos Energy.
The Sentry and the Void, from Marvel Comics, are this trope taken to its extreme. They manifested as alter-egos of a single person, representing the good and dark sides of humanity, respectively. The Sentry is a superhero and the Void is a supervillain, and in a slight variation on this trope, the Void is exactly has powerful as the Sentry, and sometimes kills someone for every person the Sentry saves. They've also split into separate bodies, and sometimes make out with each other.
ONE TIME. And immediately following this, the Sentry threw the Void into the sun. Given the Void's speech prior to this, this doesn't decrease the Foe Yay any, though.
It was later revealed that the Void exists because before becoming the Sentry, Bob Reynolds was a junkie who broke into a research facility looking for crystal meth and ended up drinking a potion that gave him the powers of the Sentry. He became addicted to that potion and the rush of being an admired and respected superhero, but deep down he was still a selfish, murdering junkie — and this expressed itself through the Void.
It was then revealed that the Void was in fact the Angel of Death. The Sentry is kind of a mess.
Stephanie Maas/Critical Maas; The only normal in a town full of superheroes actually just shunted her super powers (including flight, strength, and creepy-puppetmaster mind control) into her secondary, psychopath personality.
Incidentally, Critical Maas is undoubtedly the inspiration for Heroes' Jessica/Niki, right down to her history of abuse by her father and her penchant for calling her 'weaker' half 'little mouse'.
Jackie Estacado in The Darkness also fits this, and likes it during the early parts of the series, as it gives him more unique ways to kill people.
Mr. Negative, a new Spider-Man villain, takes this to a whole new level. He was born when a synthetic drug split a man's personality in two - the pure good side became philanthropist Martin Li, while the pure evil side became Mr. Negative. Both fight with each other in an interesting way - Negative likes to corrupt all Li's good deeds, while Li helps people that suffer because of Negative.
Curt Connors and Lizard are often portrayed this way However, there's a Retcon which reveals that Connors was always in control of Lizard and let him out when life was too hard for him. He just keeps telling himself it's not true.
Though not portrayed this way in the original comics, all subsequent portrayals of the Venom symbiote have made it out to be this, with even the comic book symbiote having been RetConned into a more malevolent entity.
In the Batman RIP storyline, Dr Hurt's mental attacks on Batman unleash a "back-up personality" Bruce had installed in his psyche, known as the "Batman of Zur-En-Arrh", who is essentially Batman without Bruce Wayne, i.e. completely unstoppable, but willing to do things the normal Batman wouldn't dream of to get results (including, it is implied, torture). Once again, more of a "superpowered" (he's still a Badass Normal) Id Anti-Hero side than truly evil.
Eclipso has acted as this to various characters in the DC universe. Subverted in one Blue Beetle comic, where Eclipso invokes this by releasing Jaime's darkest fantasy of ultimate power to serve as her champion against Paco. This backfires when this turns out to be a dentist, who is easily defeated.
Raven of the Titans has to maintain strict control over her emotions to the point of being The Stoic. If she doesn't, she runs the risk of manifesting the extradimensional demon side of her family tree and becoming Daddy's Little Villain. This is one of the major reasons Raven and Beast Boy have never been able to make a relationship work in the long run despite strong mutual feelings for each other.
The Golden Age version of the Cheetah, Wonder Woman villain, was the violent split personality of a celebrity, Priscilla Rich.
Star Sapphire, villain and Love Interest of Green Lantern Hal Jordan, was similar, with Carol Ferris being unaware of what her alter ago was up to.
Astronaut Brice Rogers became the Moon Man after passing through a comet's tail while traveling to the Moon. During the day he was Brice, but exposure to moonlight forced the emergence of his criminal alter ego.
In pre-Crisis on Infinite Earths continuity, Stacy Macklin appeared for several years in DC's Wonder Woman series. She made her final appearance in World's Finest, inheriting Moonman's powers and developing an alter ego as Lady Lunar.
Anti-hero (but firmly one of the good guys) Hellhound in Astro City's Dark Ages arc has a huge, colorful demon inside him. When it comes out, well, the demon isn't one of the good guys.
The third Black Knight Dane Whitman's first squire, Sean Dolan, suffers from this. As himself, he's a normal human Plucky Boy and Boisterous Bruiser-in-training who looks up to Whitman and wants to be a good sidekick. But when he makes the mistake of drawing Whitman's cursed sword, he turns into the demonic Bloodwraith who is willing to kill good and bad alike, cruelly mocks Whitman and his friends and can (literally!) level small countries. Sean spends much of the time afterwards trying to keep his evil side and addiction to the sword's influence under control and failing miserably.
In the Death Note fanfic Zenith, Darkness, Reverie, this is Kira's purpose. Although the "superpower" in question is Kira's utter lack of morality and the willingness to wield the Death Note. Kali is arguably the more intelligent/strategically capable personality.
It's revealed in "Playtime" that Princess Gaia was simply an illusion hiding her real Super-Powered Evil Side, the monstrous Nightmare Whisper.
Luna explains the Super-Powered Evil Side thing a bit in "AJ's Dream": it's what happens when a single aspect of an alicorn (or being with a similar power, like a wielder of an Element of Harmony) consumes becomes the only part of them. Nightmare Moon was when Luna's night became the only thing that mattered to her and Nightmare Whisper was when Fluttershy was consumed by her kindness. We're also shown possible Super-Powered Evil Side forms for Applejack (Nightmare Mirror), Rainbow Dash (Nightmare Manacle), and Pinkie Pie (Nightmare Granfalloon), though all of which are Alternate Universe versions of the mane cast.
The final stretch of the Dark World arc introduces Nightmare Eclipse, an alternate-timeline Twilight Sparkle gone Nightmare after being consumed by the desire to make Discord suffer for what he's done, and thus created a "Groundhog Day" Loop to trap him in, deleting each world and rewinding time each time Discord is defeated so he can suffer some more... and not caring that she's casually deleting/killing millions of innocent ponies each & every time she resets the loop.
In addition, the end of the Mind Games arc sees Diamond Tiara transform into a Nightmare, but as a result of Discord's psychological manipulation and breaking of her spirit rather than a facet of herself consuming her. Her theme is Discord's brand of chaos, and she earns a certain eight-pointed star symbol as her new Cutie Mark.
Sakuya inherited part of the Angra Mainyu from her mother. Perhaps because of the diluted bloodline, it manifested itself as a slightly annoying second personality rather than a mad dark mode. The family calls them Yin and Yang to keep things simple. Yang is a bookish, modest, and generally reserved girl with a small fascination with Yuri Genre. Yin is an openly perverted, conniving, and slightly sadistic Yaoi Fangirl.
Haru, by virtue of being a vampire, has his Dead Apostle side. It's more of a berserk mode, but an alternate future version of him, DA!Haru showcases what it could have made him into if he ever let it get out of control.
Touma's ten years of torture and experimentation before being rescued left him with a cold blooded alter ego called, ''The Ghost''. It isn't evil, but it has no moral inhibition, wiling to do whatever it takes to protect those he cares for.
Mana Ryougi has her alternate male personality, MANA, as per her family trait. She hates being told what to do by anyone, including herself, so she mostly ignores him. Until she has her Despair Event Horizon.
Played with in Harry Potter and the Methods of Rationality. Harry has what he calls a "dark" side, which is extremely good at plotting and seeing through plots... but that's it. Harry, who is familiar with the Superpowered Evil Side trope, is very put out when he discovers that it's no better at magic than him, and thus considers his "dark" side to be defective.
Played straight in Ah Archfall. Even before he had his Archs, Jago was a competent fighter but when things got rough, he could us his dark power side, known as "Papa Jupiter". At his most basic level, Jupiter can boost Jago's fighting power to the point where he can plaster ten grown men. At the full extent of his powers, he is AT LEAST as powerful as either the Almighty One or Lady Hild (possibly more so). Unfortunately at this level of power, he has no problem with killing people, even those he cares about (though he prefers enemies as they tend to fight back). As the story progresses, its revealed that Jupiter taking over is the only thing that Jago is REALLY afraid of. This is brought to a head during the Deity Games when he faces a mirror that reflects a person's worst fears as sees the result of what a fully unleashed Jupiter can do; he devastates the world, crucifies Lamb and Damascus, beats Lind to death with a rock and uses his knife to murder their infant son. The physic torment this causes is so great that it actually makes the mirror shatter.
In Mirrored while stuck in Voldemort's form Harry draws on the power of the horcrux. Subverted because while the horcrux gives him a significant boost it is acting in complete accordance with Harry's desires even the desire for Voldemort's death.
Nathan Wallace in Repo! The Genetic Opera is a sweet, loving, family man. Until he puts on his helmet, at which point he becomes the merciless Repo Man.
Heavy Metal segment "Captain Sternn". While under the influence of the Loc-Nar, Hanover Fiste grows into a mightily muscled hulk and smashes through all opposition in an attempt to murder Sternn. Once everyone's gone, he turns back. Sternn then thanks him for getting him out of the trial... before shooting him out of an airlock in lieu of payment.
The version of Venom in the third Spider-Man movie is Peter's superpowered evil side, and it begins to affect his personality. The catch is, Venom's just enhancing Peter's latent personality traits, so out-of-costume his evil side comes across as a massive dork, and he is generally an asshole to everyone rather than being a figure of terrible power. Now for Brock, who's already portrayed as an asshole from the beginning, it's a different story.
Most incarnations of Jekyll & Hyde in popular culture portray Hyde as inhumanly large and strong, often ape-like. (For instance, the movie Van Helsing, The League of Extraordinary Gentlemen, etc.) In the original book, there is nothing superhuman about Hyde; he simply gives free rein to his base instincts. In fact, it is explicitly stated that Hyde is smaller than Jekyll, as Hyde is the primal and egotistical side of Jekyll. In the second volume of League, Hyde acknowledges this, but explains that the longer he's given free rein, and the more impotent Jekyll (the "good side") feels, the more powerful he becomes.
The Marvel Comics version of Mr. Hyde (Calvin Zabo) is a subversion too. Hyde is indeed much stronger than Zabo in this case, but Zabo is just as evil as his alter-ego. Hyde is simply far more bestial and violent.
In Redwall, some animals (usually badgers) have an attack/split personality called the "Bloodwrath" which makes them extremely powerful, very resistant to pain... and also no longer able to tell friend from foe, meaning it's dangerous to anyone who's nearby when they go into this rage.
Oh god...the badger's gone berserk...RUN!
It Makes Sense In Context. They're going into berserker rages around smaller Funny Animals whose best option is to run - whether the badger is an enemy or one's commander.
Actually, it's not so much that they're resistant as that they can't tell they're hurt until they wake up.
In Skulduggery Pleasant, both main characters, Valkyrie Cain and Skulduggery Pleasant, have evil alter-egos Darquesse and Lord Vile.
In House of Leaves, Johnny Truant channels his Superpowered Evil Side against Gdansk Man. The way in which Truant thrashes him and how the book describes Gdansk Man's wounds afterwards has led to some Epileptic Trees, such as Johnny Truant isthe Minotaur.
'"Star Wars'': Exar Kun, Darth Vader, Ulic Qel-Droma, Darth Malak... when a Jedi falls to the Dark Side, it's time to get out of Dodge.
Beranabus, in Darren Shan's demonata series. As a result of being half demon, he was able to turn into one. He does this only once, because he's afraid that this ability is one of these, in order to fight Death. He succeeds in his goal of sending it back into the demonata's universe, but dies in the process..
Honor Harrington: Havenite secret agent Victor Cachat's Evil Side isn't so much superpowered, as it just removes all stops in his already fanatical and determined character, turning him into a Death incarnate. Which tends to scare shitless those who know him only as a naive and timid Mister Nice Guy.
Multiple examples in the Skulduggery Pleasant books. Darquesse, Lord Vile and Shudder's gist come to mind.
In The First Law trilogy, a perfect example of this trope is The Bloody Nine, Logen Ninefingers's powerful, chaotic and blood-thirsty alter-ego.
In Deltora Quest, when Steven is in danger, he transforms into/releases Nevets, who usually rips apart nearby enemies, along with everyone else stupid enough to get near him.
In the Malus Darkblade series, the title character can use T'zarkan, the daemon possessing him as this, but with a twist: each time it happens, T'zarkan gains a little more control over him.
In Count to the Eschaton, hero Menelaus also has houses the super-powered and amoral Iron Menelaus. Likewise with Del Azarchel/Iron Del Azarchel.
The Dresden Files have the Denarians, fallen angels sealed in silver coins. So much as touch the coin, and they get a free pass to the back of your mind, to convince you in any way they can to accept their power. Once you do, they go from backseat driver to Superpowered Evil Side and grant you extreme power, at the low, low cost of your immortal soul. Over time, the human host starts to become as evil as the Denarian (if they weren't already).
In Wild Cards, The Radical has the combined powers of all other alter-egos of Captain' Trips, thus making one of the most powerful aces. He's also, depending on where you stand, a Well-Intentioned Extremist or a full-blown villain. Obviously he won't let Trips take back the control of his body.
Trapped on Draconica: Halfway transforming into Mordak makes Kalak far more powerful but it also makes him a mindless killing machine.
Rachel from Animorphs has two examples: in The Separation a freak accident with a starfish morph splits her into two Rachels, a 'Nice Rachel' that's timid, ditzy and strategic, and 'Mean Rachel' who gets all of Rachel's Blood Knight instincts but none of her restraint. Later in The Return Crayak tempts Rachel with godlike power, transforming her into a steel-clawed, indestructible killing machine dubbed 'Super Rachel'.
Greg Hartmann from the WildCards universe is an amoral, egoistic and hypocritical politician that don't care about who he uses, kills, maim or hurt in his hunt for power. He is clearly an evil character, but he is absolutely lovely compared to his alter ego Puppetman, a complete sadist and sociopath that uses his potent emotion manipulation power to get the worst of everyone and only lives to revel in the violence and horror he creates.
Live Action TV
Niki in Heroes has her amoral alter ego Jessica ("ikiN"), who has sole control of her super strength until the finale, when Niki finally gets to use it for herself. She might have used it once on impulse a few episodes prior to stop an attacking policeman—this is debatable.
In the second season, it was explained that some people suffer a dissociative break to cope with the new reality of being a metahuman. When Niki conquered the Jessica persona at the conclusion of the first season finale, she manifested a third personality named Gina several weeks later.
Lori developed one in the Big Wolf on Campus episode "Manchurian Werewolf" after being bit by a brainwashed Tommy. She became a member of the werewolf syndicate. Merton also got a dose of the evil life in "Everybody fang chung tonight" and "Boy who tried wolf" where he become a vampire and a werewolf.
Kamen Rider Kabuto's Tsurugi Kamishiro is actually the Scorpioworm, who actually manages to overpower both Cassisworm Clipeuses when he was unable to defeat even one as Kamen Rider Sasword.
In Kamen Rider Kuuga, Kuga's Ultimate Form could possibly make its user become the ultimate evil and destroy the world and has a kick capable of doing said act.
The Kamen Rider Decade version of Kuuga also possesses this form but unlike the original, he remains the evil 'Black Eyed' version his entire run in this form. Taken further when he gains Rising Ultimate form which first used as a mindcontrolled Dragon by the Big Bad.
Decade gains an evil alternate form in his Violent Emotion form, which turns him into a destroyer of worlds realities.
In the live-action Pretty Guardian Sailor Moon, Sailor Moon's alternate persona Princess Serenity is a Superpowered Love Makes You Evil Side. The second half of the series is spent just trying to contain her, and upon the realization she can't be contained, at least minimize the damage she causes.
In the Farscape universe, starving Delvians start sprouting growths that paralyze any predators that touch them, turning would-be predators into prey. If that doesn't help, they'll start emitting spores, gradually paralyzing any and all lifeforms in the vicinity. In this state, higher cognitive functions are compromised, gradually turning them more and more feral. And if the said Delvian is Zhaan who was nice only due to her Heroic Willpower in the first place...
If Angel loses his soul (due to the lingering effects of a Gypsy Curse), he becomes the brilliant and formidable Angelus, widely reputed to be the most vicious vampire in history. As Cordelia Chase once queried about Angelus's evident genius compared to his heroic alter ego's, "...what is it about evil that jacks up the I.Q. points?" More emphasis on the evil, though.
The depth of this is such that Willow, the cutest, sweetest, and most innocent of the Scoobies at series start, becomes the only non-Giles member of the Scoobies to have directly murdered an innocent person. (Glory wasn't innocent, but the human Ben was)
The emphasis here is on the superpowered - Dark Willow is arguably the most powerful known being in the Buffyverse.
As time goes on, Willow seems to have her Superpowered Evil Side under control. It's more that she turns evil when she uses powerful magic, but then she turns back good when she stops. A scene in Season Seven demonstrates this effectively: while using a powerful barrier spell to protect an innocent girl from a spider demon, Willow gets the black eyes and black hair, and also calls the girl a bitch. After the demon leaves, she goes back to normal, and apologizes to the girl.
There's also Kal-El, Clark's Enemy Within who uses his powers a lot more efficiently and can even fly.
Isobel the witch becomes this for Lana in season four.
Davis Bloome in Season 8 has perhaps the ulitmate Superpowered Evil Side in Doomsday, an alien Serial Killer and Person of Mass Destruction that is trapped inside of him. The good news: it's possible to repress Doomsday. The bad news: the only way to do it is for Davis to commit murders as himself, a fact that slowly drives him insane, even as it lets him access Doomsday's power while he's in his own body.
The Incredible Hulk: In "Dark Side", David suppresses one side of his brain and succeeds only in letting his, well, dark side take over. He still doesn't kill anyone, but the usual halfway-point-Hulk-out-that-doesn't-affect-the-plot is much more interesting than usual.
Sam Winchester, Supernatural. When he taps into the demonic "special child" aspect, he is extremely powerful (telekinesis, exorcism, torturing/killing of demons, immunity to attacks by demons) but loses all his humanity and morality and becomes arrogant, violent and cold. His eyes turn yellow or black on a couple of occasions.
Bo in Lost Girl has a major Superpowered dark side, draining a whole room of people of their life force when someone is about to kill her lover.
The Doctor from Doctor Who is seen as this by John Smith, his temporary human self in "Human Nature/ The Family of Blood".
Star Trek: Deep Space Nine: In "The Alternate" episode, Odo temporarily gets a superpowered evil side after being exposed to a psychotropic gas on L-S VI. Its effects cause him to involuntarily transform into a monstrous form, driven by rage and self-preservation.
The Crow: Stairway to Heaven: In lieu of 22 non-stop episodes of a demented supernatural killer running about creating mayhem, the TV series of the film of the comic book created a personality split for Eric, where the Crow persona takes over when he becomes enraged or is under particular stress, or someone uses magic to make it happen.
This trope is a tradition in Japanese pro wrestling, where many wrestlers unveil an evil persona to face a special threat or to play mindgames with a rival, and sometimes get in storyline overcome by these personas and turn fully heel, because Evil Is Not a Toy. Among them there are Keiji Muto's "Great Muta" and his many emulators, The Great Sasuke's "SASUKE", Hard Gay's "Super HG", BxB Hulk's "Killer Hulk", Gamma's "Gamma Daio", Kikutaro's red mask, and many others.
In 1996, Japanese wrestler Jushin Thunder Liger, who traditionally played the role of a face, unveiled his "Kishin Liger" heel persona, when he wrestled heel wrestler the Great Muta, who mid-match ripped Liger's mask off, revealing his "oni face". Liger's wrestling style would become notably more aggressive and heel-like in this persona.
This was also Mick Foley's schtick for a while in the WWE: when he needed a "boost" for a match, he'd come out as either his Mankind or Cactus Jack persona.
One Tag Line for the original Vampire: The Masquerade was "A Beast I Am, Lest A Beast I Become" — leeches had to allow themselves little atrocities like controlled blood-drinking to avoid being consumed entirely by the madness of their inner blood-hunger. However, while in the thrall of such a state, vampires can ignore wound penalties, which is a significant combat asset.
Werewolf: The Apocalypse: Werewolves, on the other hand, don't tend to view their own berserk frenzies as "evil" (after all, Rage is a gift from Luna... and they have plenty of reason to be really, really pissed off). However, if they give in to their Rage too fully, they can enter a state called "Thrall of the Wyrm," just like a normal frenzy, except that it's inescapable, and tends to provoke rather horrific behavior (typically either cannibalism, rabid dismemberment, or... erm... defilement).
In Changeling The Dreaming, changelings can get magic ("Glamour") from normal humans two ways — the first is by inspiring them to do something incredible. The second is to "ravish" them — make them feel despair. One example of this involved convincing a small child that the Fae in question had just murdered the Tooth Fairy.
Wraith The Oblivion: Wraiths have got their Shadow, a second personality usually existing within the wraith trying to make them them give up their struggles and descend into nonexistence or permanently hand over control of their body. Depending on the Shadow, this might take many forms, some direct and some seemingly helping the wraith. The latter type often has ways to enhance the wraith's body when temporarily in control - better attributes, more magic powers or even items appearing only during the Shadow's catharsis. Such wraiths then always have the ability to let their Shadows handle combats too tough for them, but are generally loathe to do so because there's no telling what else the Shadow might do while in control.
Kindred of the East: Being a kind of vampire, Kuei-jin must keep their amoral lower soul (P'o) in control, lest it takes over them in a frenzy of carnage. It's not an easy thing to do since each Kuei-jin's P'o is out of balance, compared to the living. Some Kuei-jin Dharma try to make good use of this evil side, though.
In Werewolf: The Forsaken, werewolves have two degrees of rage: one where they have to kill something, and another where they have to kill everything.
Exalted has a couple instances where the characters go temporarily insane on some level:
The Solar Exalted might have this particular category nailed; one of their abilities — Ascendant Battle Visage — places them into a battle-trance that physically changes them to reflect their Battle Aura, and grants them intense and brutal combative skill. Remaining in this state too long, however, sends them into an Unstoppable Rage, at which point they become even deadlier, and fight until either they're dead or everything else that might be considered an enemy is dead.
The Ghost-Blooded (in the 1st edition at least) could learn a certain Arcanos called Lower Soul Ascendant, which fits this trope to a T — increased physical power at the cost of embracing sociopathic behavior.
And to round out White Wolf's systems: In Scion, the Scion Companion supplement adds the Tuatha Dé Danann to the playable pantheons. One of their pantheon specific abilities? The "Ríastrad" mentioned in the above mythology entry.
In Dungeons & Dragons 3.5, there is a prestige class called Frenzied Berserker. Once they've Frenzied, which is like a Barbarian's Rage but moreso, they attack until either the frenzy runs out, they are incapacitated/dead, they pass a will save, or there's nobody nearby.
Highly common in Warhammer 40,000, and one of the reasons Space Marines (Grey Knights in particular) are required to pass such rigorous and often deadly mental trials as a method of prevention. The most obvious are the Blood Angels Chapter and its successor chapters, whose members are known to succumb to a condition called the Black Rage which increases their already Super Soldier levels of strength and vitality and turns them into frothing berserkers who seek only death, often organised into aptly-named Death Companies led by skull-helmeted Badass Preachers. The Eldar however create one of these intentionally, inducing a form of split personality disorder so that in battle they can fight with unhindered skill and brutality while retaining absolute control over themselves the rest of the time. Then there's Chaos, which, being The Corruption and Always Chaotic Evil personified, fully embraces the added power this trope provides with no concern for the consequences, because "sanity is for the weak".
And then there's the Possessed Marines, who have a Superpowered Super Evil side.
Takanuva gets one of these in BIONICLE, after having some of his Light drained by a Shadow Leech.
For some reason, having an "evil/dark side" was a really popular game mechanic in the early 2000s.
Ryu, from the Street Fighter series, has a power inside of him known as the Satsui no Hadou, or Surge of Murderous Intention, which he works to suppress and was retconned as the reason that Sagat received the scar he bears on his chest. Akuma (Gouki in the Japanese version) represents someone who has fully given in to the Satsui no Hadou, and when he's not holding back, he is truly fearsome. In Street Fighter Alpha 2 and 3, Evil Ryu is actually a secret, playable character (a dark-skinned, red-eyed version of Ryu with some additional attacks that was borrowed from Akuma).
Ryu's Superpowered Evil Side is a result of his being unable to control the power. However, since the power still involves a willingness to kill and a more brutal mindset even when in control, he makes no effort to even try to learn how.
Evil Ryu returns to the SF storyline through Super Street Fighter IV, where he's a playable character. This time he has gone from merely a dark-skinned, red-eyed Ryu to a borderline Person of Mass Destruction with a kanji carved on his flesh and a glowing orb where his heart should be. ACK! And actually, making Evil Ryu return to the surface was a plot point in the Ties that Bind tie-in anime. It almost works.
In Prince of Persia: The Two Thrones, the Prince occasionally transforms into an alter ego with a bladed chain-whip (and the skills to wield it gruesomely, out of nowhere) and who can completely regenerate whenever exposed to the Sands of Time, but at the expense of constantly deteriorating health otherwise. The Dark Prince is also a sociopath with a massive entitlement complex. Even when in the Dark Prince body, the Prince remains in control, but the Dark Prince hints that the health draining out of you is actually going into it, and that dying of this will give the Dark Prince control of the body (and, incidentally, Babylon).
Soma Cruz in Castlevania: Chronicles of Sorrow inherited Dracula's soul, while Alucard is his son (sort of). The emergence of their Superpowered Evil Sides are Alucard's greatest fear and Soma's bad ending.
The Slayer from Baldur's Gate II, a ten-foot tall spiky mass of pure murder that the protagonist ends up gaining access to. Using it results in a reputation loss, except for scripted events that force you into it. Plus there's the whole thing about it killing you (and thus ending the game) if you stay in that form for too long.
Dizzy from Guilty Gear has a similar problem with one of her sentient wings. She's VERY powerful and unleashing her true powers causes her to lose control to her evil wing, Necro, who's motto is "Destroy everything!". With Dizzy being a Friend to All Living Things this really troubles her.
Not that the other wing, Undine, is much better. Unlike Necro, she won't attack anyone who comes within reach randomly, but she will annihilate anyone she perceives as even the slightest threat to Dizzy without even a second thought... and her definition of "threat" is just a little bit too broad for Dizzy's tastes. Undine is the wing that unleashes Dizzy's version of Gamma Ray... that says it all. Oh, and her tail has a will of its own too and often eats anything within biting reach.
Rozalin from Disgaea 2: Cursed Memories manifests a Superpowered Evil Side during the storyline when her seal is broken, which turns to be her former life as Overlord Zenon, complete with both memories and power, back from the dead.
From La Pucelle Tactics, Croix has his Superpowered Evil Side as the Dark Prince serving the fallen angel Calamity.
Iori Yagami in The King of Fighters is normally amoral, and (judging from his winposes) has quite a few screws loose. Riot of the Blood Iori, however, has absolutely no sense of self-restrain and will attack anything on sight.
Leona also fits this trope, sometimes even more perfectly than Iori, because she is in the "good side", while Iori can't decide himself; with some Emotionless Girl spices added, though that said, she has smiled once◊.
Jaster Rogue in Rogue Galaxy does this, although his "evil" side isn't so much evil as so far above everyone else that he barely notices they exist. Hard to tell the difference from the "normals" point of view, though.
Johnny Garland gains this far into the third Shadow Hearts game. Johnny actually died at a very young age along with his sister, Grace. Their father managed to get his hands on the series' Tome of Eldritch Lore, in hopes of resurrecting them. He only succeeds in reviving Johnny, as his sister gives up her humanity in order to save him, leaving her an empty shell, "Lady". Because Johnny's body is composed of both "Malice" (negative energy) and "Will" (the human soul/spirit), after being fatally wounded by Killer, his "true" Malice form, the Awaker, emerges.
In Xenogears, one of the primary villains for the first half of the game, Id, turns out to be an alternate personality of the primary hero, Fei. Id was completely different from Fei in appearance (except for the face itself) and had several magnitudes more power than him. Shown in a flashback to defeat a half dozen Gears, unarmed, with a single hit each.
When Id manifests, even Weltall gets a superpowered evil side, transforming into Weltall-Id.
This is subject to the variation mentioned at the top of the page. Even though Id doesn't get as much screen time as his alter-ego, Fei is a personality built on top of Id and the Coward (the original personality). Id is able to access the memories of the Contact, which Fei is only able to do near the end of the game.
Grahf plays this role in two ways. He is the metaphysical alter-ego of Lacan, a previous incarnation of Fei, and the (current) physical alter-ego of Khan Wong, Fei's father.
Zero of the Mega Man X series gains several powerful new abilities when he is exposed to large quantities of The Virus. When he is fully awakened by the Sigma or Zero Virus, he loses all of his emotions and his personality shifts to the one of an cold blooded killer.
In Mega Man Starforce Pat has a personality disorder. He'll randomly go into an evil state named "Rey" and punch someone. This foreshadows him fusing with Gemini and becoming Gemini Sparks W and B.
Elena's alternate personality Millenia from Grandia II.
Anti-Sora from Kingdom Hearts II. Ironically, while Anti-Sora looks damn cool, he's not actually much more powerful than Sora in terms of offense, and can do nothing other than attack solo in a frenzy of clawing and kicking. The inability to heal, gain experience, or really do anything else makes this form a liability, and it can't be entered deliberately.
Riku, on the other hand, plays this trope straight in four of the games, but primarily in Kingdom Hearts: Chain of Memorieswhen you can play as him and "Dark Mode" is one of his key abilities.
In Metroid Prime 3: Corruption, Samus gets the ability to enter Hypermode, fueled by the evil substance known as Phazon. Staying in this mode for too long corrupts her, leading to a Nonstandard Game Over, specifically turning into another Dark Samus.
And then they added a Super Powered Light Side along with Dark Jak in Jak 3. You use both to solve puzzles.
The Lost Frontier gave one to Daxter as well, although the bunny-ears are not good for its badass credentials.
Sonic the Hedgehog has one in Sonic Unleashed. At night, he'll turn into Sonic the Werehog, a bulky werewolf-esque form of himself with super strength and stretchy arms. Though his personality remains more or less the same except for the obligatory howling.
This exception is lampshaded after it is revealed that Chip is actually Light Gaia, a benevolent spirit, who is guarding the earth. Sonic then asks, if it could be Chip who keeps him from succumbing to Dark Gaia and becoming evil when in Werehog form, to which Chip replies that he has nothing to do with him, it's Sonic's kind heart alone which lets him keep his sanity.
Sonic and the Secret Rings also introduced Darkspines Sonic, which was the result of sonic absorbing the secret rings of anger and sorrow. Notably, Darkspines doesn't just beat his enemy, he actually crosses the line Sonic usually doesn't and beats him to death.
Jinpachi could be said to be another example, as he holds the King of Iron Fist Tournament 5 to find somebody to stop him, as he could feel the Devil taking over. He also sends a letter to Wang, his old friend, to have a go as well.
Jun Kazama counts as well. Turns out she's Unknown, a superpowered... well, where she sits on good and evil hadn't been established, but she basically turns into Venom and becomes a little more dangerous.
In the early Soul Calibur games, Nightmare was supposedly a Demonic-Sword-induced version of this for Sigfried. The latter freed himself in SCIII, but his armor and residual evil power was too attached to Soul Edge to die.
Two far more definitive examples show up in SCIII as the boss Abyss (spawned from Zasalamel's quest for the power to die), and bonus boss Night Terror (Nightmare on 'roids).
Fighters Destiny has a clown character, and there's a cheat you can use which turns him into a werewolf.
Primal has its main character who personifies this when in any of her monster forms.
It is revealed in .hack//G.U. that Natsume has a split personality: Her normal self is her cheery, slightly ditzy self while her other self is crazed, rampaging Pker known as The Edgemaniac. This is played for laughs.
Haseo's Avatar, Skeith, is also portrayed as this, although Haseo usually does a better job of controlling him than normal for the trope. Skeith is seriously scary when out of control, his actions being the only thing in the games that actually seem to frighten Haseo.
Vincent Valentine from Final Fantasy VII can transform into a variety of monstrous forms using his limit breaks. Possible forms include Frankenstein's Monster, Jason Voorhees and Chaos, the Big Bad of the first game in the series.
Terra's Esper form seems to be this in Dissidia: Final Fantasy, as when she goes into it during the plot, she suddenly loses control and ends up fighting the other heros.
Django, the gun-slinging hero of Boktai. In the second game of the series Django is infected with vampirism and almost loses his soul before his half-brother Sabata purifies him with the Solar Pile. After this, Django eventually gains the ability to transform between human "Red Django" and vampire "Black Django." Though it might be argued that Black Django isn't that much more powerful as, despite his dark-based powers, he loses the ability to use all of Django's magic (gaining new spells only he can use) and is harmed by both rain and sunshine. In the third game, the transformation is changed to a "Trance" attack, where he becomes much more powerful, however the transformation only lasts a limited time, or until you leave the room.
The Terror Mask in the Splatterhouse series embodies this trope, especially in Splatterhouse 3 where its own personality, and its effect on Rick, become more apparent.
In The World Ends with You, the Bonus Boss is the Noise form of Hanekoma, and since he really kills you if you lose, it can be seen as this. The fact that, during the fight, he spews his human form's inspirational phrases like battle cries could show that he's not quite in control.
At one point in Wild ARMs 2, Ashley is transformed into some kind of zombie-like creature early on, but before he is taken completely, he makes it to the Argetlahm and pulls it out — this has the result of countering the demon within him with the good force of the sword, and the resulting combination is a badass black-armored-red-trimmed entity called Knight Blazer. He can't actually use it in battle until a few plot points later, but once he gains the ability, nearly every remaining boss fight in the game boils down to letting him transform in order to open up a can of whoop-ass. At one point, in a moment of Unstoppable Rage fueled by a terrorist threatening his girlfriend, Knight Blazer actually takes another level in badass and transforms into a gold-armored knight with extra abilities that is that much more powerful. And then at the end of the game, the "demon" portion escapes Ashley's body and reveals itself to be the Cosmic Horror responsible for devastating the world centuries ago, Lord Blazer. This leaves Ashley with the holy sword. Guess who wins theresultingboss fight.
In Mega Man Battle Network 4, if MegaMan turns dark enough from the use of dark chips, his dark side will appear after he is K.O.'d and start fighting. MegaMan gains control back after a while.
Chaos Shadow Soul in Battle Network 5 made MegaMan completely invincible and uncontrollable as the AI attempted to duplicate the player's fighting style. Speaking of which, the fight against Dark Rock in Battle Network 4 did the same, using every chip the player had ever used even the navi summon chips and giga chips and also can use all the program advances you done up to that point when you fight it and unlike the player version theres no warning when it uses them. it tends to favor using these.
The Suffering initially plays this straight with a "primal form" that shreds enemies but damages your Karma Meter. However, it's subverted twice over. First, you learn that you're merely "under the delusion" that you turn into a monster when you go berserk. Then, the final boss proposes to "cure you"—and it sort of works. In the sequel, you can transform without harm, even if you're playing as a good guy.
The Fierce Deity Mask ("Oni Mask" in Japan) in The Legend of Zelda: Majora's Mask turns Link into this. It helps that oni means "demon"note Okay, it's more like "ogre" but let's not get into semantics.
In Painkiller, when you collect 66 souls of enemies you killed, you transform into an unstoppable demonic killing machine. It only lasts for about 30 seconds but that's all the time you'll need to obliterate an area full of monsters just by looking at them.
It may not be on quite the same level, but in Pokémon Colosseum, there's a random chance that a Shadow Pokemon might go into 'Hyper Mode' during a battle. The Pokemon in question will be surronded by a black aura, and will do more damage - BUT. It won't be able to use any moves you've unlocked, it can't be healed, and it will also take recoil damage when it attacks. Thankfully, you can call out to it, which brings it back to it's senses.
Playable characters Dante, Lucia, Nero, Vergil and Trish from the Devil May Cry series all can transform into a demonic version of themselves with stronger attacks and health regeneration. In Devil May Cry 2 this becomes easy, with the characters' demon forms not being thrown back, halted or even slowed by enemy attacks.
Ryu from Breath of Fire 3 has the Infinity gene, which unleashes his ultimate dragon form Kaiser. When used by itself, the player has no control over Ryu, and he attacks at random (including allies!). Averted however, when combined with the trance and radiance gene. Using the flaw gene makes also the kaiser form controlable, but it is severely weakened.
In the same game you have also the Weretiger Form of Rei, which rises his attack power to high levels but he will also attack at random the party members. The problem with random attacking can be solved with the "Influence" skill. It can be learned from the Boss Goblin enemy.
In the same regard the "Berserk" Skill is identical to the Weretiger form, but it comes along with the downside that it kills you in three turns, it can be adverted with a Soul Gem or with the armor Manly Clothes though.
In The Warriors, every playable character has a "Rage" meter of varying length. When it fills up, the character can use a highly effective radial attack and their grapple specials become a great deal more violent, usually capable of outright killing an opponent regardless of health. Later on, you can even trigger it by using a healing item with full health. Subverted, though, considering that most playable characters are criminals of some sort.
Every single main character in Persona 4 must be recruited by defeating their Superpowered Evil Side, or more accurately, their Shadow Archetype: the embodiment of all the things about themselves they can't stand to admit. The shadows have grown immensely powerful out of all proportion simply because being denied makes them stronger; once accepted, they become much weaker and "tame."
The series also gives us Black Frost-which is the horrific result of adorable Mascot Mook Jack Frost remembering he's supposed to be a demon after all. The process apparently renders him awesomely more powerful... and much more evil.
As said in comics, any game that has the option of the black suit has it as this. In games such as Spider-Man 3 and Spider-Man: Web of Shadows, his attacks become more violent and agressive, usually with Combat Tentacles and spike tendrils.
Subverted in Spider-Man: Shattered Dimensions, in which the Ultimate Universe Spidey (the one who actually has the Symbiote) is the kindest of the Spideys, he does have a Rage mode, however (the only one with one) in which he becomes more scary looking and his attacks are more devastating, but his comments are actually the oposite. He'll say things like "Yeah, lets have some good old fashioned RAGE!" and "Raaage! I'm not actually angry, I just like saying it." But it's justified, since Madame Web is using her powers to keep the suit from taking over.
This is the reason the quaggan have survived in the Post Apocalyptic world of Guild Wars 2 despite being Actual Pacifists; actually manage to piss them off, and they turn into what is best described as a cross between an orca and a bulette, with absolutely no regard for their safety or collateral damage. And when they calm down, they'll apologize for it.
Mu-12 is this for Noel Vermillion. Mu is her original form.
In Puyo Puyo Fever, Klug has one in Strange Klug and Sig might have one in Black Sig. But we do mean "Superpowered" for Strange Klug; he's not only a final boss, but some kind of demon king from long ago. He appears often due to, surprise, Klug'sAesop Amnesia of carrying a possessed book after it already possessed him.
Noire and Vert from Neptunia, when they activate HDD. The Lastationian CPU becomes more brash and competitive upon transformation, while the Leanboxian CPU gets more arrogant.
The Hero of Shining Tears is normally a timid amnesiac with a dislike for fighting, but when somebody else puts on the other matching ring of power he turns into 2 superpowered sides, 1 being a moral knight of justice while the other being a insane, Omnicidal ManiacBlood Knight.
Many characters in the Nasuverse have an "inversion impulse", the desire to go psycho and unleash the full power their bitchin' superpowers on the world in an orgy of murder, due to their monstorous bloodlines.
In 'DRAM Atical Murder, Aoba has the ability to control other people and enter their minds with his voice alone but the more he uses this ability, the more a darker, more powerful side of him that wants to destroy everything in its path threatens to consume him completely. In the True Ending, though, it's revealed that his "evil" side was actually more lonely and misunderstood than evil and happily remerges with Aoba after Aoba fully remembers and accepts him.
Roger in College Roomies from Hell!!!. Unleashing his werecoyote side is often the Roomies' only way out of a tight spot, but he has trouble controlling it, and he's been warned by another werecoyote that sooner or later he'll be unable to change back. He actually has a second werecoyote form that is a stronger embodiment of this trope.
Graveyard Greg of the Gaming Guardians has two separate and unrelated evil sides. Not only is he a werewolf, with an animalistic but self-aware wolf personality, but he's also possessed by a voodoo spirit which can take control of him at times. Greg has retained control by pitting them against each other, until they decided to team up to suppress him, becoming a zombie wolfman which calls itself "Mange".
In Jack, the eponymous character, a personification of the sin of Wrath and the Grim Reaper, will occasionally enter a kind of "Wrath Mode". In this state, he will be overcome by his sin and gain a boost of power. He later absorbs and seals the sin of Lust, who talks to him, hassles him and will sometimes help him in fights through his cloak.
In Gnoph, Will's symbiotic partner Lyss usually acts as a Bond Creature; but when he passes out and she takes full control of his body, she acts like an example of this trope, due to being Ax-Crazy and having no compunction against killing (or eating) humans.
Subverted in The Order of the Stick: It initially looked like the comic was going down this path when Vaarsuvius made a deal with three fiends in order to save the lives and souls of his/her family. Having successfully rescued them from a dragon, (s)he then proceeded to use epic magic to slaughter as many as a quarter of the black dragons in existence, in what was widely held to be a Moral Event Horizon by the audience. The next strip reveals that, contrary to what the devils said earlier, the power-up had no effect on Vaarsuvius' ethics and that whatever atrocities (s)he just committed were all of his or her own doing. The fiends had lied to her in order to induce a nocebo effect.
Black Mage encounters himself in the Castle of Ordeals, where the Light Warriors have to face symbolic representations of their own flaws. However, the only thing that could represent Black Mage's flaw (Pure Evil) was himself, as nothing else was evil enough. Then it turned out that even his own duplicate didn't quite represent some of the more evil things he'd done, which corrected itself as the real Black Mage ran through them by becoming even more evil (as well as powerful). He then absorbed his own evil and power back into himself, eventually unleashing it to destroy four other incredibly evil beings, whose own evil and power he then absorbed, giving him an evil aura as well as incredible magical power.
Later omniscient superwizard Sarda (a.k.a. The Omnipotent Jackass) has absorbed Black Mage's superpowered evil side. Lampshaded thus:
Sarda: Who knows what kind of sadistic and impossible stuff I'll do now.
Trace Legacy of TwoKinds has one of these. Whenever he gets extraordinarily pissed off, his memories of his old life cause a person shift. However recent comics have seemed to show this side is merging with is current persona. (As the racist evil side is shown defending Keidran, which is something only his normal persona would do.)
Played with in Homestuck: both of Gamzee's personalities are Ax-CrazyOmnicidal Maniacs. They're just kept merged and stable due to Gamzee's addiction to sopor slime. Rose's temporary grimdark transformation may also count.
Franken from Noblesse slips back to his Mad Scientist persona when he unlocks his power, which allows him to curb stomp ABSOLUTELY EVERYONE. His Slasher Smile has to be seen to believe it.
In Goblins,Dies Horribly has a magical, green arm that acts as a sort of Empathic Weapon most of the time. But when Dies is severely threatened, the arm tends to take over his body and slaughter everything in the immediate vicinity in creatively gruesome ways.
In Roommates Goblin King Jareth has one. Technically it's his original personality, but his love and gratitude for the girl he choose, the camaraderie he feels for his friends and his own desire to change makes him suppress it. Not that it helps much, even his "Good" side is an "easily entertained but frequently bored trickster" (great intentions but sucks in this whole good thing) the "Evil" one is an "amoral ruthless fae lord" (Fourth Wall Observer meets The Fatalist and his role in the story tends to be: Villain).
A fairly goofy example occurs in episode 40 of Red vs. Blue: in order to fight off a horde of "noobs" in Battle Creek, Caboose gets in touch with his "angry side", which results in him gunning down the noobs while shouting bizarre threats like "I will eat your unhappiness!" and "Your toast has been burned, and no amount of scraping will remove the black parts!"
Caboose: Now I'm thinking of a kitten... covered in spikes... that makes... me... angry... YYAAARRRRGGHHH!
Caboose jumps off the roof he was hiding on, shaking the ground when he lands
Red Zealot: It's the beast! The Anti-Flag! Come to live among us and rule for seven years! The end is nigh- blleergh!
He did this by replicating the effects of being possessed by the AI O'Malley, who gives whoever he inhabits a Super-Flanderized evil side, which in Tex's case makes her super aggressive. When he inhabited Grif he wanted to Take Over The World, but was too lazy to do anything. Only Church is immune to this effect, being as cynical as it is possible to be already. Among such reasons as Omega being the embodiment of Church's rage. Church already has a Hair-Trigger Temper, but the Omega personality was formed back when he had lost his wife, and was Jumping Off the Slippery Slope in remorse. When Church is seen after producing Omega in a flashback, he's little more than an Empty Shell who doesn't even remember her. The main villain of the series embodies Omega's rage too, having originated them, and the evil he does is on a far grander scale than anyone else.
In Tales of MU, the side of Mackenzie that some fans have taken to calling Kenzie is two details off from being a textbook example of a Superpowered Evil Side: First, Kenzie is triggered by hunger instead of desperation in battle. Second and more distancing, the closest thing to a superpower Kenzie has over normal Mack is being willing to hurt people that are trying to kill her.
In the Global Guardians PBEM Universe, Josiah Brimstone is a skilled and powerful sorcerer, but when his spells are not enough, he can unleash his inner demon on the bad guys. The inner demon doesn't always stop at beating down the bad guys, though, so Brimstone is really reluctant to do this. Viviane von Klause, the "Witch of San Francisco" tries to avoid overusing her magical abilities because if she goes too far and pushes her magic too far, the ghost of her Satanist grandmother takes over for a little while.
The same thing happens to Rarity in the comics, making the "superpowered" bit all the more obvious as she is able (with some help) to provide a serious challenge to the rest of the cast put together.
Dragons are prone to greed, and the greedier they get, the bigger and stronger they get, which makes them even greedier. In "Secret of my Excess", a few extra gifts on his birthday awaken Spike's dragon greed, and he becomes a huge rampaging full-grown dragon taking everything he wants from Ponyville. He snaps out of it when he is reminded of his moment of generosity earlier in the episode.
In the Teen Titans cartoon, a splashing of chemicals prompts good-natured vegetarian Beast Boy to turn into a surly, meat-chomping bad boy who later turns into an anthropomorphic wolfman that was capable of taking on the entire team (as well as another wolfman). Later it's explained that, instead of turning Beast Boy into an actual lycanthrope, the chemicals only unleashed what was already there (which makes no sense considering that they also changed Adonis), and he's given an antidote so that he won't transform against his will. He once again manifests this hidden wild side in a battle against Slade, slicing through a number of his goons before Slade puts a stop to it with his own supercharged powers.
Interestingly enough, both of the times that Beast-Boy assumed this form, he did so in an attempt to protect Raven. Coincidence?
Possible subversion: It's made quite clear that "Man-Beast" isn't evil, just a really powerful, rather intelligent animal. Like any animal, it becomes hostile when scared, but generally it keeps to its human form's wishes. Beast Boy still expresses concern that it's still inside him at the end of the episode.
He does seem to have acquired a degree of control over it later on, though he rarely uses it. In its first appearance, though, the Beast is more like the Hulk—not evil, but uncontrollable, volatile, and dangerous.
There's Raven's "Daddy Issues". Dr. Light should really have backed off like she told him.
Kim Possible: Ron Stoppable is ordinarily a likable goof, but he's been zapped a couple of times with Applied Phlebotinum with superpowered evil results. The first time, the device was malfunctioning and gave him a transfusion of Dr. Drakken's evil, turning him into the insane, fiendish, blue-skinned, cleverer-than-Drakken-by-a-long-shot "Zorpox the Conqueror." The second time, the device was a properly-working instant Mirror Morality Machine. One might simply chalk it up to a removal of inhibition, as Ron has some serious mystical mojo that backs him up at critical instances. The point is, Ron's evil side is inexplicably hyper-competent to the point of being slightly unnerving (enough to cow Shego into obedience) — he mastered the art of having multiple plots within what may well have been hours of his conversion, and showed himself to be a serious, focused martial artist, at least the equal of Kim. Granted, he HAS had secret ninja training...
Ron loses focus easily, his evil side doesn't. When Ron gets focused, he becomes competent. But where did his evil side get the skills to invent that plasma catapult thingy?
That time he had Drakken's evil, which probably included his Evil Genius (Hey, it makes as much sense as having it include his blue skin).
Unlikely, as Drakken still had his intelligence and Ron has invented things before, such as the battle chair he built out of garbage when Drakken kidnapped him. Not to Mention Ron was intelligent enough to repair the Attitudinator after he was turned back to good.
He may be Book Dumb or Brilliant, but Lazy— for example, the doom machine that Drakken forced him to build actually worked and was made in a CAVE!! WITH A BOX OF from scrap. And in a cutscene from the game A Stitch in Time, kindergarten-age Ron is capable of giving a complex speech. Word of God actually suggests that he may have repressed his genius to fit in better.
He channels negative emotion into action a lot better than positive happpy emotion. Taking over the world is a negative emotion. He also channels his powers when someone hurts Kim. He just does a lot better when in angry mindframe.
Megatron clearly learned his lesson from his ill-fated attempt at turning Rhinox evil in Beast Wars. He placed a Restraining Bolt in Tankor that would immobilize him if he ever tried to directly attack Megatron.
Another example involving Rhinox shows up in Beast Machines when it was revealed that Tankor, The Brute tank that had been hounding the Maximals all season was really Rhinox in a sort of Brainwashed and Crazy situation. Sadly things got even worse after Optimus Primal freed him from Megatron's control and he turned against his former allies and started an Evil Plan to take over Cybertron.
Some poor, misguided fools seem to have heard that G1 Grimlock is capable of combining with the Dinobots to create "the Beast," a terrifying titan of destruction and primal rage. These people are damned liars. Or at least, weren't aware that the sheer suckitude of that Story Arc was so much that Unicron and the Autobots teamed up to wipe it from space and time.
A literal form of this is The Batman version of Clayface. Just using his powers turns him insane and monstrous.
In Earthworm Jim, whenever Jim's puppy sidekick Peter is injured, he turns into a hulking, ravenous mutant whose sole goal in life seems to be beating Jim to a pulp. In a joke inversion, Peter's Evil Twin is evil when "normal", but his psycho mutant form turns out to be good.
The 2006 revival of Biker Mice from Mars has Stoker turn into an insane rat-like beast called Stoker rat.
An episode of Mr Meaty had Parker (who is normally calm under pressure because he just blissfully takes all those insults people kept throwing at him in) turn into a Hulk-like monster due to being unable to hold all those insults in any longer.
In an episode of The New Batman Adventures (the one after Batman: The Animated Series), Batman inhales an anti-fear toxin made by the Scarecrow. As a result, Batman has no fear. NONE. Bruce Wayne (in disguise) infiltrates some facility and has to deal with a moat with an alligator minutes after being exposed. He walks through the moat, becoming completely submerged. While still under water, he sends the animal flying from the moat and continues on his merry way. This later worsens as he loses his fear of killing...
Distinctly more of an Superpowered Id than Evil Side, but Freakazoid! is like this to mild-mannered nerd Dexter Douglas.
Coop on Yin Yang Yo. In his regular form, he's weak and nerdy. However, when he's infused with The Night Master's power, he becomes a match for Yin and Yang. Later on, Yin also gets a evil side to her when some of the Night Master's power corrupts her.
When audiences found Invader Zim's GIR cute, the writers created an episode where Zim actually fixes him - and he becomes suddenly destructive and hyper competent, determining the only way to conquer Earth is with Zim out of the picture...
A flashback in episode 19 of Generator Rex reveals that Rex can turn into a savage biomechanical Giant Robot Evo. One of the reasons White Knight is so hard on Rex is because he is afraid of Rex becoming that monster again. Rex himself has no clue he can do this thanks to his amnesia.
Avatar: The Last Airbender : The Avatar. When the vessel for the Avatar is threatened it locks into a superpowered mode that controls all the elements; it cares only for the protection of the current Avatar and to uphold the balance of the world. In a proper one-on-one battle, it is nearly impossible to deafeat.
In Winx Club, Lord Darkar uses his dark magic to turn Bloom into her Superpowered Evil Side in the form of Dark Bloom, who is Bloom with a black version of her fairy outfit and demonic yellow eyes. And he does it twice with her in the second season. Once to have her ship off Alfea's Codex piece to him through his Team Pet Kerbog, and again to have her help him open the portal to Realix with the four Codex pieces where he can acquire the Ultimate Power. Both times had her turned back to normal in the end - for the first time, the impostor Professor Avalon used his wand to lift the spell, and for the second time, it was Prince Sky who changed her back through his words of kindness, especially his "I love you" line, after which she returns the Ultimate Power to its rightful place to Darkar's shock and dismay.
One episode of Xiaolin Showdown was about Dojo transforming into an evil monster and destroying everything in his path after Omi accidentally frees him from his cage.
The Shadow Thief from Justice League Unlimited was eventually revealed to be the Superpowered Evil Side of Carter Hall, who subconsciously willed him into existence out of a desire for a villain for him to defeat. He later attempts to kill Green Lantern so that Carter can be with Hawkgirl.
In an episode of The Real Ghostbusters, an accident causes Slimer to start turning into a huge, demonic, bestial version of himself that the heroes name "Big Green", who sort of becomes type of Split Personality. Once they find out what happened, Egon manages to separate them into two ghosts, and then defeat Big Green without hurting Slimer.
Adventure Time has the Ice King as an odd, somewhat tragic, and not-so-evil example. It's not immediately apparent, but when his backstory is revealed it turns out he used to be a normal, sensitive guy who discovered an ancient crown that gave him magical powers which allowed him to survive The End of the World as We Know It. Unfortunately, it also drove him more and more insane each time he used it, until, by the time of the series, he's locked into his Crazy/Evil Side full-time and can only vaguely recall his own origins and who he used to be.
Mike of Total Drama Revenge of the Island has an extremely comical case of multiple personality disorder, then in Total Drama All-Stars it's revealed that he has a sinister, Matthew Patel-like personality named Mal (also known as "The Malevolent One") that his other personalities have previously kept at bay. The question of whether Mike is their body's original personality or Mal is the original and Mike is Mal's Helpless Good Side is brought up, but not resolved.
The nigh-mythical Norse berserker rages, an adrenal and psychosomatic overclocking which gave one the ability to ignore mortal injuries but removed one's ability to distinguish friend from foe. The less brave soldiers simply drank a lot of alcohol.
However there is little consensus as to how effective the rages were, or if they even existed, some historians attributing these accounts to overwhelming fear of the Norseman, a likely scenario considering the Norse's tendency to attack and loot monastaries (being non-Christian and thus seeing no importance in them other than being very wealthy), which was the feudal equivalent of crossing the Moral Event Horizon.
While these rages probably didn't turn them into the unstoppableengines of destruction some accounts make them out to be, Vikings having them is at least plausible, artificially induced or otherwise. Even today we have people who will have "red-outs", when they become so angry they lose all inhibitions on their rage, and generally just want to break and hurt everything around them and generally go violently insane. They tend to not feel pain in this state, the adrenaline and anger block it out. This matches up with certain accounts which list Berserkers as immune to "blade and fire", which makes sense in that while those things hurt like a bugger, it takes hell of a lot to have them put you down even semi-permenantly quickly if you can't feel the pain (decapitations and heart piercings aside) short of lopping off a leg or intentionally severing a tendon(both harder than they sound), whereas a blunt weapon can put you down pretty easily by crushing joints or breaking bones, generally making motion impossible. As someone who has experienced "red-outs" (and incidentally is of Viking descent if you go back far enough) when a person goes into a red-out, little things like pain, identities of people, etc. simply don't matter compared to the need to act on your anger. All this of course isn't even factoring in hallucinogens and other drugs, which demonstrate daily their ability to send people into a berserkeresque rage.
Dissociative Identity Disorder (alternately called multiple personalities) can cause the "removal of moral restraints" version.