Or they'll be like a fusion of the first and second example and decide that killing/destroying everything is the ONLY way to save EVERYONE from the pain/pointlessness of existence.
They might withdraw from society, become a hermit or drunkard, and ignore the ongoing state of the world. If the current generation of heroes meet them, the fallen hero will mock how their deeds are useless. Most likely, however, they will help the new heroes in the hopes that they won't suffer the same fate.
Yu-Gi-Oh! GX: Judai's Heroic BSOD causes him to transform into the villainous Supreme King once he decides to use evil's methods to fight evil itself, complete with a literal gaggle of Fallen Heroes (evil versions of his normal Elemental Heroes).
In Yu-Gi-Oh! 5Ds, Rex Godwin qualifies. Before he became the tyrannical head of Security, he was the legendary Mysterious D-Wheeler who tried to build the Daedalus Bridge and connect Satellite to Neo Domino, hoping to free the residents of Satellite from the oppression of the upper class. But Security had other ideas. When they tried to arrest him, he drove his D-Wheel off the incomplete bridge, vanishing, becoming a martyr to everyone who followed him. In a sense, when he became a Dark Signer, he was a Well-Intentioned Extremist, who sought to put a stop to the conflict between good and evil that seemed doomed to repeat itself endlessly. When he was finally defeated, the Daedalus Bridge was completed as a modern bridge, and stands as a symbol of the man he once was.
Prétear. Along with the revelation that the Princess of Disaster is the current form of the last Prétear, Takako, the show also plays with the possibility that anyone who becomes the Pretear could become the Princess of Disaster. Which naturally leads Himeno Awayuki, the current Prétear, straight into a Heroic BSOD while she sorts it out.
Jyu Viole Grace from the second season of Tower Of God, the identity of the grown-up former hero and Wide-Eyed Idealist 25th Baam from season 1. After being betrayed and nearly killed by the person he looked up to and trusted the most as well as experienceing several other traumatizing events he grew frustrated from the system that fulfilled some people's dreams while crushing the last hopes of many other. As a result, he joined the fanatical religious crime syndicate FUG, known enemies of the Zahard monarchy, making himself a public enemy. He became an Anti-Villain, destroying other peoples dreams and aspirations as he trained himself to kill the King and his retainers, many of whose relatives he had befriended in early years.
In G Gundam, Gentle Chapman was one of the most admired Fighters ever, but after old age and an illness caught up with him, he could only win via cheating (perpetrated without his knowledge by his wife). He paid for it by dying...and was revived as a zombie by the Devil Gundam.
Master Asia also falls under this. He was formally a part of the Shuffle Alliance, a group that was devoted protecting humanity from self destruction, and Domon Kashu's mentor. But during the previous Gundam Fight, he saw how badly the Fight damaged the Earth and blamed humanity for despoiling their homeworld so carelessly. After meeting the Devil Gundam, he joined its plan to wipe out humanity, believing that it was the best path for life on Earth to survive.
Also Devil Gundam. It was built as nanomachine colony to restore Earth's environment, but Ulube wanted to use it as a weapon. When it escaped to Earth, the impact messed with its A.I and turned it into an Eldritch Abomination.
Gundam 00's Big Bad, Ribbons Almark, seems like this, considering that he was a Gundam Meister who piloted the very first Gundam in the series. It's actually a subversion, as he was never a hero. The intervention when he saved Setsuna had no glorious goal beyond a field test of the 0 Gundam, and he was supposed to kill all witnesses. He only spared young Soran Ibrahim because he saw the devotion in the young soldier. And this incident triggered his god complex, leading him to become the Big Bad.
Gale Raregroove, Pumpkin Doryu, and Captain Hardner in Rave Master.
In Naruto, Nagato, aka Pain was born with a special power that marked him as a literal messiah. Inspired by the words of his childhood friends and mentor, he desired to bring about a lasting peace. However, his pacifistic attempts left his friend Yahiko dead and him crippled, shattering his dream. So, he set out to create a new peace, by sharing the pain that he had suffered.
Hanzo, his predecessor as the leader of the Hidden Rain Village, also started off with good intentions before he became a Crazy Survivalist.
Belive it or not, even Orochimaru qualifies. Although he'd die before admitting it, there was a time, before his defection, when he legitimately cared about his teammates and fought on behalf of his village. After seeing many people die in war, he became obsessed with gaining immortality and lost any sense of ethics.
Oskar Von Reuenthal in Legend of Galactic Heroes. Particularly tragic, since he did not rebel against Reinhard Von Lohengramm out of genuine malice or ambition, but because he was framed and was too proud to accept punishment for a crime he did not commit. His death is arguably the most senseless and undeserved in the entire series.
Ubel Blatt has a number of these, similar to the Overlord example further down the page. 14 heroes go to fight the Dark Lord. On the way there, 3 die heroically, and upon the entrance of the Evil Lair, 7 of the survivors get cold feet and stay behind. The other 4 go to defeat the villain, winning with great difficulty. Returning home, those heroes get ambushed by their former comrades and are bloodily murdered. They go on to become the "Seven Heroes, who defeated the Dark Lord and the '4 Lances of Betrayal'". The 7 Lances end up going insane with power, but can get away with anything because of their hero status. Though one of the 4 is back and wants revenge.
After having his alignment changed by the effects of Nirvana, it emerged that Hot-Eye of the Oracion-Seis was one of these. He originally just wanted to raise enough money to fund a search for his lost brother, Wally, but seemed to have gotten corrupted by his team mates somewhere along the line. After all is said and done, he calmly accepts incarceration for his deeds while he was a villain, after finding out that the Fairy Tail guild members had met Wally not too long ago, and declares himself happy with the knowledge that he's alive and well.
Jinno from Afro Samurai. He was Afro's best friend as they trained together for years, but one night under the Bodhi Tree, he witnessed all of his friends and fellow pupils killed by bandits, and then Afro coldly murdered their master to obtain the No. 2 headband. He blames Afro for everything that happened, and had his ruined body fitted with cybernetics to turn him into a formidable warrior. Before Afro can challenge Justice, he has to fight past his childhood friend.
Lady Flair in the Dirty Pair Flash Universe, a Former WWWA agent and predecessor to the Current Lovely Angels who after a mission goes bad and felt betrayed went on to become an assassin , but makes an heroic sacrifice after realizing the truth of what happened.
In The DCU series Justice League Task Force, the hero, Triumph, was lost for decades in a time warp, and returned to join the Task Force as its leader. His difficulties in adapting to the new times, added to the desertion of Martian Manhunter and Aquaman from the TF's ranks (which caused the government to close it down due to the real Justice League returning), ended with him broke and being harassed by common thugs. With the help of a Heroic BSOD and a pen that contains a powerful genie, he wreaked havoc and mentally dominated his former allies into fighting the JLA. When he failed, he was frozen screaming.
It's worth pointing out that it's HEAVILY implied that Triumph is being influenced by the genie, and therefore arguably not this trope. Also, he sees the error of his ways and repents shortly before being frozen. And, in the interest of completeness, there's also the point espoused by his old writer that the inadvertant loss of his soul (long story) left him Not Himself.
About a million alternate future stories depict either Batman or Superman as this, frequently with the other one trying to pull them out of it.
The Long Halloween, the series that inspired Nolan when he was writing the script for The Dark Knight, has a Harvey Dent that worked alongside Batman and Commissioner Gordon. We later find out that Harvey may not have even been responsible for some of the deaths, it may have been his wife trying to end all the terror that was happening and trying to get Harvey to come back. A closer inspection reveals plot holes with this revelation, and it's vague whether she did it or was just crazy. This is only one version of Two-Face's origin, but all the ones worth mentioning show him as working with Batman before turning into Two-Face.
Both Sinestro and, later, his Arch-Enemy, Hal Jordan, are Green Lantern Corps members who turned evil. Sinestro wanted to enforce order, so he became a Knight Templar dictator of his home planet, Korugar. Years later, after seeing his home city nuked, among other things, Hal Freaked Out and destroyed the Green Lantern Corps and tried to remake the universe. Hal was later retconned into being possessed by the Anthropomorphic Personification of fear itself, and Sinestro was influenced by a demon telling him a prophecy that Korugar would destroy itself if order wasn't enforced.
At the beginning of Kevin Smith's Daredevil run, Karen Page was stuffed into a fridge. Then, Brian Michael Bendis took over and his identity was exposed to the public, and eventually, he was incarcerated for obstruction of justice. After beating the rap, his new wife, Milla, suffered a psychotic breakdown and the marriage dissolved (Matt cheating on her with Dakota North happened in between). By the time Lady Bullseye started to kill his closest allies to resurrect them as zombie ninja slaves, Matt finally said "screw this" and abandoned his life as Matt Murdock to become leader of the Hand. Though still a Technical Pacifist, the crossover Shadowland might soon change this as Marvel is now promoting Daredevil as the new "greatest super-villain of the Marvel Universe".
Astro City has El Hombre, an Expy of Batman from Los Angeles. Though he became prominent in the super-hero circle, he became upset at his lack of respect from the populace and his love interest's marriage to someone else. He then hired a super-villain to build a robot to attack the city so he could stop it in a high-profile fight. He was betrayed by the villain, and when it was later revealed that El Hombre commissioned the attack, he became a wanted fugitive and disappeared into his civilian identity.
Decades later, he tries a similar ruse, killing low-level supervillains to unite their ilk against him, eventually gathering them all in one place, and wiping them out in his new heroic identity as El Guerrero. His former sidekick, Bravo, while being ashamed of El Hombre's actions, still holds a great deal of respect for the great man and the hero he once was.
Shakara - the Big Bad responsible for most of the destruction has recently been revealed to be Cinnibar Brenneka.
Richard Dragon, most famous for being the best martial artist in the DCU, was one of these for a while until Bronze Tiger dragged him out of it.
Hank Pym becomes one of these (usually of the retired variety, but occasionally the Anti-Villain version) every couple of years when something bad happens to Jan and/or Ultron does something horrible that he blames himself for.
Superboy Prime. He begins his career battling the Anti Monitor, the DC Universe's greatest threat, and is immediately forced into Limbo with his home universe destroyed. After years in Limbo, he returns, convinced that Earth's heroes are screwing it all up and ultimately decides that this universe needs to be replaced by a better one.
Supergirl, specifically the Linda Danvers incarnation, may or may not qualify, as she ends up in Hell off-panel and there are veiled references made to her having done something bad but we never find out what. Ironic in that Linda had a large hand in redeeming another Fallen Hero, Twilight, who had been using her healing powers for evil for centuries before Linda brought her back to the side of the angels.
Irredeemable centers around The Plutonian, a Superman-like superhero who snaps violently after a long and thankless career and proceeds to become the irredeemable Big Bad set to obliterate the world that he once protected. Inversely, there's the spin-off Incorruptible, which focuses on former Supervillain Max Damage who, in the wake of The Plutonian's rampage of destruction, decides to become a hero.
Irredeemable is written by Mark Waid, who co-created Triumph (mentioned above), and has confirmed that a lot of the original ideas behind Triumph (who Waid wrote very little of) ended up in Irredeemable.
Sin City has Jack Rafferty, who was once a hero cop but eventually degraded to the level of an alcoholic Bastard Boyfriend. Word Of God states that his story will eventually be told.
In the My Little Pony: Friendship Is Magic alternate continuity Legends Of Equestria, this role is filled by Princess Celestia, who slowly goes mad from the anxiety of having to constantly watch out for threats to her reign, and the psychological trauma of her internal struggle with the dark magic she accepted in order to live forever and create the cutie mark spell.
Princess Luna also falls victim to this, as she becomes the Knight Templar Nightmare Moon, hoping to put an end to the excesses and abuses of freedom that her sister has caused. In the end, Celestia decides to eliminate her sister rather than accept her help with ruling the kingdom.
In the Pokemon fanfic Pedestal, Nick becomes one of these after learning that his brother was killed.
The Pony POV Series has a number of them. The first we see is in "Epilogue", a Bad Futurewhere Discord won. The Mane Six have been turned into his immortal Co-Dragons and now do his bidding, at best having no idea they were once heroes like Twilight Tragedy and Rarigreed and at worst knowing they were but having no way to escape their fate. Though Liarjack still remains somewhat heroic, as she does her best to save lives, but she still has done horrible things like helping kill Queen Cadance. Thankfully, the Dark World Series finally sees them rise again.
Nightmare Mirror, Applejack's Alternate Self who became a Nightmare after her Applebloom didn't escape the events of Story of the Blanks, turning into a truth obsessed Multiversal Conqueror intending to rid the multiverse of deceit. Thankfully, Applejack manages to team up with five other alternate versions of her (including Liarjack, now back to Applejack) to purify her with the Elements of Harmony, then Applejack and Orangejack convince her to turn good again. The same journey also shows Nightmares of Rainbow Dash and Pinkie Pie called Nightmare Manacle and Nightmare Granfalloon respectively.
One alternate universe shows the Harmony Queens, a version of the mane six who went Knight Templar, overthrew and imprisoned Celestia and Luna, and took over the world eradicating anything they viewed as Disharmonic and brainwashing ponies in mass with the Elements. According to Word Of God, they're based off the Justice Lords from the Justice League.
Finally there's Nightmare Eclipse/Paradox, the true Big Bad of the Dark World Series. She's a potential future version of Twilight who went Nightmare to take revenge on Discord for what he did. She then become She Who Fights Monsters and trapped Discord in a Groundhog Day Loop, erasing billions from existence each loop to feed her grudge and not even caring anymore so long as he suffers.
Harvey Dent in The Dark Knight definitely counts, after he becomes Two-Face. Formerly idealistic, he grows steadily more cynical in the face of the Joker's crimes and, after the Joker's Breaking Speech, turns into a Nietzsche Wannabe who believes that Chance is the only fair law.
Kirsty Cotton in Franchise/Hellraiser: Hellseeker.
Transformers: Dark of the Moon: Sentinel Prime, by virtue of Face Heel Turn. He made a deal with the Decepticons that would restore their home planet of Cybertron. Additional material establishes Megatron as this. Before the Great War, he was the Lord High Protector of Cybertron, ruling equally with Optimus.
Skywalker may have been the most infamous Jedi to turn evil, but he was hardly the only one. Count Dooku, the villain in Attack of the Clones, was once a member of the order too, and Expanded Universe books often feature "Dark Jedi" as villains, many of whom were formerly real Jedi, who are too numerous to list.
Madmartigan, the master swordsman in Willow, qualifies for the "lost himself in dissipation" version of the trope.
Morgoth is a fallen Valar and this universe's equivalent of Satan, which as noted above is one of the oldest examples there is, so he counts. Sauron too, he used to be a Maia which were like angels.
Ineluki, the Storm King of Tad Williams' Memory, Sorrow, and Thorn, started out as a hero of the Sitha, but he took his people's racist tendencies to the extreme and, when Asu'a was sacked, he turned from defending his people to killing everyone else. Also an example of Motive Decay.
In the The Belgariad, Zedar, The Dragon, used to be Belzedar, one of Belgarath's sworn brothers and a servant of the god Aldur. When Torak, the Big Bad of the series, struck Aldur and stole the Orb, Zedar headed out to confront him...and found himself overwhelmed by Torak's power. Faced with The Dark Side, he gave in to his barely-suppressed lust for power and swore fealty to Torak.
Gerald Tarrant/The Hunter in the Coldfire trilogy. Sorcerer, philosopher, and Prophet of the One God, until the religion he had created excommunicated him, at which point, he killed his wife and children as a part of a bargain with Dark Powers.
Agent Denton, who is also He Who Fights Monsters. When Harry soulgazes him, he sees that he used to be a genuinely good man, but his methods (most notably, his use of the Hexenwulf belts) made him just as bad, if not worse, than the people he's trying to take down.
The Corpsetaker was once a member of the White Council.
Kalthused of Within Ruin. He starts out as a heroic leader fighting for the independence of his country but when his wife Ankaa dies he falls into a spiral of despair. He forsakes all his old morals and plunges the country into futile wars for centuries in order to bring Ankaa back to life.
By the end of AnimorphsJake has become one of these. It's stated in the epilogue that the only reason he's not being tried as a war criminal is because he fought for the side the won the war.
Trapped on Draconica: Pre-series Kazebar was Draconica's number 1 humanitarian. To reward his good work Dronor granted his son the power to travel between worlds, believing that if any human deserved this honor it would be Kazebar's family line. Whether he was tempted by this power or if he was Evil All Along is not made clear.
David Gemmell loves subverting this trope. Waylander is a war hero. Then his family is slaughtered, and he makes sure none of the assailant "takes less than an hour to die", and finances his vengeance by becoming a professional killer. Then he seeks redemption, gets it somehow, his family gets killed again, vengeance ensues, redemption again, and then he dies a stupid death by the hand of the son of the man whose death was the reason for his first face heel turn toward good. Gemmell does not like black and white.
Buredoran/Brajira from Tensou Sentai Goseiger. It's revealed that he used to be a Gosei angel who turned on the rest of his team and stole there powers killing them. He uses an experimental power to travel 10,000 years into the future to enact his grand plan to remake the world and become it's messiah and savior.
Jon Mitchell from Being Human. He tries hard to fight his vampire urges and tries to be an example of reform, but he falls off the wagon in season 2 and slaughters a train of 20 people. He never really gets back to normal after that and commits suicide.
Linderman. His low-key, evil approach is made all the more monstrous when viewers realize that, having the ability to heal most injuries, he chooses to have people killed, kidnapped, and crippled instead.
Adam Monroe. He is introduced as Takezo Kensei, the literal hero of legend. Despite trouncing all the fantastic tales attributed to him in one fell swoop, Kensei proves himself a true hero many times over during his time with Hiro - only to do a Face Heel Turn when Hiro steals away the woman he loves right out from under his nose. Four hundred years later, his heartbreak has driven him to seek a 'second chance' by wiping out 93% the world's population.
In the French fantasy dramedy Kameloth, the Knight Lancelot start out as the noble and charismatic hero we expect him to be, but he has always been ideologically opposed to the libertarian policy of Arthur (who he considers a proof of weakness) and considers himself more worthy of the holy mission given to his king. After the spoofed-legend-opposed-got-away-with-Guinevere-part, he openly rebels against Kameloth's order and became the tool of a dark sorcerer named Melangeant, who presents himself as The Chessmasteringanswer of the gods to Arthur's failure in his mission
Eli David in NCIS seems to be this. In a way, he's reminiscence of Denethor.
Some of the best episodes of Scrubs deal with this happening to Dr. Cox. While the fall is temporary, the sight of the normally caustic and extremely confident physician in tears is very heartrending, to say the least.
Lex Luthor from Smallville is a great example. He started off as nothing more than a good Samaritan friend to Clark Kent. As time went on, he became nastier and more cynical at the world, and possibly became Clark's worst enemy. However, how long would the show actually last if Luthor was kept a good guy throughout the entire show?
Jack Bauer in 24. First seven seasons? Someone who pushed himself ten times beyond the brink both physically and mentally to repeatedly ensure the safety of the country and world. Final season? After his latest mission winds up going horribly wrong and ends on a tragic note, he winds up embarking on a personal crusade of revenge that ultimately causes an international crisis and nearly instigates a war that would lead to the deaths of millions of innocent people, just barely stopped himself after realizing how bad those repercussions would be. Rivaling him would be Tony Almeida who went from protecting people to threatening them all to avenge the murder of his wife... and unlike Jack he never had a Heel Realization.
LokiLaufeyson (you know, the badguy in Thor and TheAvengers), the blood brother of Odin and best friend and Guile Hero sidekick of Thor, started off as a light hearted comic relief of sorts, with something of a running gag in the stories of him being threatened with death by the other Gods (sometimes in retaliation for a prank, sometimes because they're just jerks like that), then his attempts to fix everything resulting in him suffering some form of mutilation or humiliation (from having his mouth stitched closed to being raped and impregnated by a giant horse), but he remained loyal to Asgard, but eventually, his humiliations and repeated sufferings, combined with the fact that the God's didn't particularly care for him that much, made him bitter and resentful, until coming to a head when they imprisoned him on a boulder (chained up by his own son'sentrails) with a giant serpent dripping venom into his eyes. Once free of this Fate Worse Than Death, Loki lead the enemies of Asgard against the Aesir, dying in battle against Heimdall, but in doing so, brought about the End of the World as We Know It.
What makes this even more tragic is that this aspect is often left out in adaptations, where Loki ends up becoming the Norse equivalent of a standard bad guy. Since Loki's Face Heel Turn happened right before he died, many later versions of the Norse tales have him instead being a real asshole who constantly screws over the others. In fact, the original reason Loki was chained to the boulder was retconned, so instead of being because he insulted them all at a party they didn't invite him to (he wasn't invited so they could talk about him behind his back) was instead because he arranged the death of Baldur (something he merely claimed to do in order to wind them all up), with no party or comments involved.
Hulk Hogan's infamous Face Heel Turn and transformation into Hollywood Hogan in WCW's Bash at the Beach was born of the realization that he was "old news", and that the fans he had lived his whole life to please weren't really interested in him anymore, which he just couldn't stand. After all, he's Hulk Freaking Hogan, the biggest icon in wrestling! Maybe the fans didn't deserve to cheer for him! Maybe they deserved to have him and his buddies from up north destroy everything about WCW that they enjoyed instead!
Similarly, Chris Jericho's recent WWEFace Heel Turn was fueled by the fans' continued cheering for Shawn Michaels — who was not only a lying, cheating hypocrite, but was unrepentant for having retired the great Ric Flair. In Jericho's mind, it's not him that turned heel; it's the fans.
Speaking of HBK, he's been this several times (all versions at different points), including presently.
Specifically, he was the outright villainous version during his initial turn after turning on Jannetty, the anti-hero version for parts of the initial DX run, the anti-villain in his crusade against Hulk Hogan, and the retired/disinterested version during his various retirements and sabbaticals.
Subverted with Bret Hart in 1997, who only turned against the American wrestling fans, but was still considered a hero in the other territories.
Stone Cold Steve Austin's character was defined by his drive to become the WWF Champion "at all costs". Usually, this just meant that Austin would theoretically work harder than everyone else to get it. His Face Heel Turn came out of taking that to the logical extreme, where he allied with his perpetual nemesis, Vince McMahon (and his rival, Triple H, the next night...who tried to kill him in the past), at the now-famous Wrestlemania X-7, to guarantee he would leave the event as WWF Champion.
When The Rock fought heel Hollywood Hogan at Wrestlemania X8, he unexpectedly got a lot of boos and "Rocky sucks!" chants (he was also booed at the last Wrestlemania, but that was against Stone Cold in Texas, so it's understandable). Given how popular The Rock is supposed to be, it came as a shock that people would boo him over the heel, Hogan. The Rock later used this as partial reasoning for turning heel the next year where he defeated both Hogan AND Austin in back-to-back PPV's.
Mick Foley was probably the most famous "hardcore" wrestler during his stint as Cactus Jack, due to his runs on WCW and his Death Matches in Japan with Terry Funk. However, when he made his Face Heel Turn in ECW, he cited the fans' expectations of the wrestlers (and their desire to see wrestlers put themsleves in increasingly dangerous situations) as the reason he turned on Tommy Dreamer, the heart and soul of ECW. He then began his "anti-hardcore" gimmick where he became a WCW-shilling, non-hardcore butt boy for Eric Bischoff, everything ECW fans hated in wrestling.
Chris Benoit was at one time considered arguably one of the greatest wrestlers of all time but his claim as one of the greats has all but been erased due to the events of the last days of his life.
The Blackguard class from Dungeons & Dragons is specifically designed for fallen heroes, allowing the player to "trade in" Paladin levels for Blackguard levels after completing a Face Heel Turn. However, while the character may not have been planning on the transition, the player almost always is; the Blackguard class has pre-requisites that don't make sense for most Paladin builds.
Could also be Fridge Brilliance. A Paladin attempting to interact with an evil outsider peacefully would likely develop into a much sneakier character than a normal Paladin, something so unusual, considering their training, that it would be difficult for them to learn.
The idea is more that a character won't go straight from Paladin to Blackguard, but will instead "fall" as a Paladin (losing all their Paladin-specific abilities but retaining their raw stats) and then choose to pursue the path of the Blackguard instead of redemption, summoning an evil outsider to teach them how to do so. The most awkward pre-requisite for a fallen Paladin is still the five ranks in Hide, however.
Yu-Gi-Oh gives us the Gigobyte / Gagagigo / Giga Gagagigo / Gogiga Gagagigo cards, which describe a young troublemaker who has a Heel Face Turn upon having his life saved, and in trying to gain enough power to help repay his debt, he acquires cybernetic upgrades which eventually eat his soul and drive him mad. It's a surprisingly detailed story told not only in the flavor text of his own cards, but in illustrations for other cards that otherwise have nothing to do with him. It's only natural that his story gets played out in one of the video games. Said video game had him realising the error of his ways. This almost certainly qualifies him for Face Heel Revolving Door.
And nowHe's Back, recovering his heroic soul and getting new armor that lets him keep the power he was searching for.
Warhammer 40000. Exactly half of the Primarchs turned against their father the Emperor of Mankind in the great betrayal of the 31st millennium. Each of the ten traitors had a personal reason for turning their back on their father. None are more tragic than The Paragon Horus the Warmaster (i.e. the Emperor's second-in-command, since he was the Emperor's favourite son).
The Chaos Gods showed Horus a vision of a terrible future where the Primarchs are gone, the ideals of the Great Crusade are forgotten, and the Emperor is worshiped as a god in a brutal fascist dictatorship. Horus dealt with the Chaos Gods and turned on the Emperor (the man who saved humanity and rebuilt civilisation after a horrific dark age that lasted thousands of years) to save humanity from this dark fate. The Horus Heresy results in the Imperium becoming increasingly authoritarian due to its paranoia over Chaotic rebellions like Horus's, and the final battle leaves Horus dead and the Emperor in a coma, unable to steer the Imperium onto a more enlightened path. Fast forward ten thousand years, and Horus's actions have caused the Imperium to become a brutal fascist dictatorship where the Emperor is worshipped as a god and the Imperial Truth (the atheistic rationalism which the Emperor personally believed) is thought of as heresy, since it offends the immortal God-Emperor. Just… damn.
A Fallen Hero or a Misunderstood one? Alpharius and Omegon, the twin Primarchs of the Alpha Legion seemingly sided with Horus against the Emperor. However, they did this after being informed by an universal alien organisation that when the Emperor would defeat Horus, the universe would continue in constant warfare against the Chaos Gods , ultimately devouring the universe in the proces. They were told that when Horus would defeat the Emperor, the Empire would fall into chaos for 2 or 3 generations after which the Fallen Primarchs and Horus, ridden with guilt of their actions, would seek war upon themselves and destroy mankind and the Chaos Gods in the proces (which is what the Emperor was trying to do). Upon reflecting this information the Alpha Legion decided to side with Horus against the Emperor to actually follow the path he had directedfor destruction of the Chaos Gods. Thus they became Fallen Heroes to take the action a Right Hero should do.
In Exalted, this is how new Abyssals are made: a Solar is captured, strapped into a Monstrance of Celestial Portion, and tortured until they die, become catatonic, or become an Abyssal. There's nothing that prevents them from breaking loose, trying to rise again and setting off on a quest for redemption back into a Solar...
In the Suikoden series, you'll usually recruit a couple of these per game. The most prominent is probably Geddoe from the third installment, who, in an interesting twist, in addition to being a Fallen Hero (retired/disinterested variety), is also one of the three main protagonists.
In the Halo Universe, Mendicant Bias betrays the Forerunners, who made him with free will (which is what caused this) and then had him communicate with the leader of the Flood, The Gravemind, to the Flood. Then the Forerunners build Offensive Bias, who lacked free will, to defeat him. He did, but it was too late. The Forerunners had to activate the Halo Installations, killing all life in the galaxy. Offensive locked MB on the Ark. All the beings were cloned and seeded on their worlds (well, mostly, a few mistakes were made where some beings got placed on the wrong planet, as humanity found a few planets inhabited by humans that nobody knew). Cut to 100,000 years later, and Mendicant Bias causes Master Chief and Cortana to go somewhere unknown to, as he said, show his masters that he had atoned for his sins. There is a short history at the end of the Forerunner/Flood War, with an explanation of where John is going (sort of).
Also, Hash in the medieval chapter, who is a mild case of this. He was a hero who defeated the Demon Lord, but lost faith in humanity and chose to live as a hermit on a mountain. He subverts it by helping Oerstred defeat the Demon Lord again.
In Overlord, your enemies are seven Fallen Heroes who represent the Seven Deadly Sins: Melvin Underbelly the halfing (gluttony), Oberon the elf (sloth), Goldo the dwarf (greed), Sir William the paladin (lust), Jewel the thief (envy), Kahn the warrior (wrath), and the Wizard (pride).
A more literal example would be the titular character, who was originally a hero who fought alongside the other heroes, but fell from a great distance and was left for dead by his companions. The Evil Plan of the old Overlord sees the main character eventually revived by his minions and given the position of the Overlord as well as command over the minions.
In the sequel, Queen Fay becomes a Fallen Hero after her Heroic Sacrifice. Florian may also be one, although it's unclear if he was ever truly a hero.
Rose: Power, it always corrupts. Gnarl: Hah, that's half the fun!
Archimonde and Kil'jaeden, the leaders of the eredar who sold their souls to Sargeras and became the leaders of the Burning Legion.
Similarly, the human prince and paladin Arthas eventually resorted to the cursed blade Frostmourne to slay the demon that was (apparently) behind the plague that turned people into the undead. As a result, it took his soul and turned him into a death knight loyal to the Lich King (who had all that planned from the start).
Then there's the death knights that followed him, which constitute Orders of Fallen Heroes. A force of them in Wrath of the Lich King are sent to wreak havoc in Northern Lordaeron... All of which seems to be little more than a ploy to lure out Tirion Fordring, one the few living beings that could even be considered anything close to a threat to the Lich King. They were just a diversion and, eventually, as a result of a climactic battle in which Tirion reveals the truth of their betrayal and ultimate expendability, pull a Heel Face Turn. The player plays through this entire sequence, including all the irredeemable evil goodness inherent therein.
And the night elf Illidan, trying to fight fire with fire (or demons with demon magic), eventually became a semi-demon himself.
The Frozen Throne shows how the arrogant-but-decent high elves turned into the evil, demon-following blood elves they are in World of Warcraft.
In a desperate attempt to save his people, Kael'thas turned to demons, and let himself be consumed by their fel magic.
Neltharion took every one of the black breed of dragons with him. They are hunted and mindlessly killed, sometimes just for sport. The truth is that they have all been driven completely insane and/or have lost every last one of their morals. The breed has almost been wiped out or forced under ground. It didn't help that Deathwing is dead, which probably just made things worse for the breed.
It might be easier to just say that every Warcraft villian fits into this trope.
Sarah Kerrigan in Starcraft. Though she didn't "fall" so much as "was thrown, had her sense of morality surpressed", once she got her free will back, she decided that she liked being evil.
It was more like her sense of morality and compassion were surpressed, allowing the darkness within to become dominant. Blizzard confirmed that Heart of the Swarm's arc will be about whether she will fall to darkness forever or transcend it and achieve redemption for her sins. So far, the trailer shapes her to be an Anti-Hero.
The Overmind itself, which was long ago taken over by the Dark Voice.
The first Diablo has King Leoric, who was strong enough to resist being completely possessed by Diablo but was left an insane and murderous wreck by the ordeal. The second game has all three of the original game's heroes; the Warrior was manipulated into becoming Diablo's new host, the Rogue became Blood Raven, and the mage became The Summoner. And in the third game, apparently, every Diablo 2 character except the Barbarian will be Ax Crazy. Blizzard is in love with this trope.
Fain, of the Red Masque from Lusternia. A brilliant and popular leader amongst the Elder Gods, he resorted to increasingly extreme measures to combat the Soulless Ones. Ultimately, he and his co-conspirators began devouring other Elders to imbibe their essence and power, and were banished to the Void. Driven insane by thousands of years of isolation, he is profoundly unhappy by the time he returns to the real world.
Ace Hardlight from Ratchet: Deadlocked. Ace was once a great hero before being kidnapped and forced to participate in Gleeman Vox's deadly gameshow, Dreadzone. Ace eventually became seduced by the thrills and infamy of the tournament, and became the deadliest contestant on the show — and The Dragon to Big Bad Vox.
Clank: I do not understand. What sort of hero would kill other heroes for money? Hardlight: Not money, tin man. Fun.
Also, Captain Qwark...for a given definition of 'hero'. In the firsttwo games he's a fame-hungry showboater willing to endanger innocents for his own glory, the third game reveals that he did once, however incidentally, save the galaxy once from Dr. Nefarious. He gets better in later games, as in he's less willing to endanger innocents on purpose and prefers to take credit for Ratchet and Clank's adventures.
Sephiroth in Final Fantasy VII. Especially evident during Crisis Core and its glimpses of Sephiroth's pre-fall personality — although cool and aloof, he was actually a pretty nice guy and hero-grade material before the Nibelheim Incident. His Dissidiaopponentswill not stop talking about this in their pre-fight quotes, especially in Duodecim. The crazy thing is that Sephiroth will alternate between calling himself one or declaring himself a general destroyer of life. "Taste the blade of a hero.", indeed.
It's funny: the heroes will either question how he could have turned or how he was ever a hero, and the villain quotes (particularly the Emperor) make it sound more like a Never Live It Down moment.
Square Enix plays with this heavily in Dissidia. Many of his quotes are contradictory to his villainous nature, such as saying "Fear not." or "Do not despair."
Seymour in Final Fantasy X. Subverted in that it turns out that he was never a "hero" in the first place.
The Nameless One from Planescape: Torment might qualify, with some of The Atoner thrown in. Apparently, he did something that was so bad in his first life, that he thought he'd be damned even if he did nothing but good for the rest of his life, and thus sought immortality in order to have more time to atone.
Big Boss of Metal Gear Solid was originally a quirky, cheerful, affectionate, paternal sort of man, who ends up going through a major Break the Cutie routine in Snake Eater, Portable Ops, and Peace Walker. He then ends up creating The Patriots with other "fallen heroes", Major Zero, Sigint (a.k.a. DARPA Chief, Donald Anderson), and Para-Medic (aka Dr. Clark, the head of the Les Enfants Terribles project and the one who turned Grey Fox into the Cyborg Ninja), along with Ocelot and EVA and creating Outer Heaven and Zanzibarland to plunge the world into eternal war before meeting his end at the hands of his "son", Solid Snake, which was prophesized by The Sorrow and Elisa.
Anti-Villain: The kind of 'peace' that his enemies sought was one ruled by a global totalitarian shadow state, and the eternal war that he sought was the opposite of their attempts to control and regulate the dangers of individual human will. Big Boss may have become a threat to world peace and security, but both were inextricably tied to imposed obedience, which is ultimately slavery.
The 7 Heroes of Romancing Sa Ga 2. Warriors who saved the world, but were betrayed by the people they saved and cast into Hell through dimensional magic.
Matriarch Benezia, having unwittingly lost herself to the very madness that she sought to stop, the ultimate tragedy being that she can't be saved.
As of the second game, Liara seems to be the anti-hero variant of this trope in the making. She gets better.
Depending on your choice of background and alignment, especially if you change alignments between games, Shepard can be played as a fallen hero.
A few characters see Shepard as a fallen hero in Mass Effect 2, no matter how you play, given that s/he's forced to work with a terrorist group.
And s/he is definitely seen as this at the beginning of Mass Effect 3 given the terrorist connection and that s/he was forced to kill over 300,000 people to slow down the Reaper invasion. One news report early in the game even refers to him/her as "the disgraced Commander Shepard".
The second game also gives us Rael'Zorah. Tali specifically fears her father being seen as this by the quarian people after he chooses to run weapons test on active geth prisoners in order to advance the cause of retaking the homeworld.
Thorndyke in Soul Nomad & the World Eaters, a Knight in Shining Armor goes this route in the Demon Path, initially submitting to The Main Character in order to save his son. As time goes on, he is forced to do worse and worse things until he is tricked into believing that he killed his own son, turning him into an Ax CrazyBerserker. When he later sees that his son is alive, Kanan convinces him that he never went mad and killed because he truly enjoyed it, finally breaking him.
If we're talking Demon Path, Revya is possibly the biggest Fallen Hero of them all.
Mithos (second type), Kratos, and Yuan (both first type, one in service of Mithos and the other one opposing him), from Tales Of Symphonia.
Ghaleon from the Lunar series qualifies quite well for this trope. He has a glowing reputation at the start of Lunar The Silver Star (and the remakes) for heroism alongside the famed Dragonmaster, Dyne. His Face Heel Turn sends the world into a panic. In the remakes, he is a Well-Intentioned Extremist who sees the Goddess Althena's decision to leave humans to their own devices as abandonment. So, Ghaleon starts plotting a way to restore divine leadership to the world. And who is the new divine leader? Ghaleon! His purpose in Lunar Eternal Blue turns out to be redemption for this, though the player doesn't learn this until right at the end of the game.
Jon Irenicus from Baldur's Gate II counts. Prior to his exile from Sulldanesselar, he was an upstanding citizen and powerful mage. Pride was his downfall; he was exiled and stripped of his soul for using his power to try and achieve godhood.
Aribeth de Tylmarande from Neverwinter Nights and Bastila Shan, Revan, Malak, and the entire Revanchist movement from Knights of the Old Republic. How you play both games determines whether the spoilered characters stay evil or not.
Also, Yuthura Ban on Korriban, who was a fallen Jedi padawan with similar motivations to Anakin when he started out (i.e. go back home and free all the slaves), who then suffered from Motive Decay and became just another power-hungry Sith.
Akachi the Betrayer and a third of his Crusade from Neverwinter Nights 2: Mask of the Betrayer are all Fallen Heroes, the dragon and the army of undead having been evil to begin with. Arraman might also qualify, depending on your character interpretation.
And the King of Shadows from the original campaign, whose mission to defend the Illefarn Empire suffered severe Motive Decay when he escaped from his extradimensional prison and found that the Empire crumbled to dust millennia past.
Skies Of Arcadia features Ramirez, The Dragon to Lord Galcian. He's described as having once been pretty similar to Vyse - artistic, kind-hearted, and loyal. Unfortunately, he was raised in near-isolation by the most arrogant culture in the game and coming into contact with the meaner parts of Arcadia proved a little much for him.
Disgaea 3 has an example of its own in Super Hero Aurum. He was originally a hero who fought some of the greatest villains his world has ever known, but the more he fought, the further he fell towards obscurity, which he feared more than anything else. He needed to relish in being known as a hero, so he began doing worse things over the years, up to and including killing a nice guy Overlord and raising his son to be a general asshole Overlord just so he could be a hero again. As Sapphire put it, he eventually "ignored being the hero".
Beldr from Devil Survivor is Baldr, god of light and beauty from Norse Mythology. After he became trapped in the underworld as a giantess refused to weep for him, he became determined to spread lament on the Earth until everything cries.
For extra bonus points? It's implied that said giantess was actually Loki.
From the same company, various hints have been dropped by Atlus saying something happened to YHVH to make him what heistoday.
Malin Keshar from Battle for Wesnoth attempted to use necromancy to defend his home village of Parthyn. However, after being rejected by his own people due to the bad reputation that necromancy has, he becomes the apprentice of Darken Volk, and begins to despise everyone more and more until he's a full-blown Villain Protagonist.
In the first game, Captain Blue is the one masterminding to escape from Movieland to take over the real world, having lost his stride twice. In the real world, he was hailed as revolutionary director, having created several good movies, but then he lost all of that. He just wanted to create more heroes. He was then somehow sucked into one of his films, and he lived all of the great adventures he wanted, but then he figured something out: the world was Too Good to Last, he started to want revenge against the people of the human world. Thankfully, he got some sense knocked into him.
May not count, but the second game gives us Jet Black, who wanted to become a film maker to show his son what a true hero was. He then found the Black Film, which started to eat at his desires, eventually twisting his desire to make a film about heroes to actually wanting to be the hero, and was going to take over the world. Again, he got some sense knocked into him.
Mathias Cronqvist, friend to Leon Belmont, brilliant strategist, noble Crusader, and genius alchemist. The death of his wife Elizabetha shattered his faith in God and he became obsessed with obtaining immortality so that he could curse God forever. And thus was Dracula born.
Teyrn Loghain Mac Tir from Dragon Age: Origins. In the prequel novels he slowly becomes a high-ranking officer for the rebels, and is later hailed as a noble and one of the greatest war heroes along with King Maric as they win and drive out the Orlesians. Sadly, he becomes so protective of the kingdom he's fought for, that he gradually turnsextremelyparanoid, and goes as far as to leave Maric's son, the current king, to die in the battlefield when he suspects him of trying to sell Ferelden out and framing the Grey Wardens, triggering one of the game's two main plots.
Although many people forget this part of her backstory, Meredith from Dragon Age II was also this trope, as she became Knight-Commander after the previous viscount of Kirkwall, Perrin Threnhold, tried to expel the templars from the city. (While not everyone looks upon the templars fondly, Perrin Threnhold is generally regarded as a tight-fisted tyrant who attacked the templars first.) Her paranoia over blood magic drives her utterly mad.
Anders, one of your party members who was once a heroic and kind-hearted Grey Warden, loses his battle with the demon of Vengeance inhabiting him and blows up the Chantry.
In Red Dead Redemption, Dutch Van Der Linde was, according to John Marston, an idealistic romantic who was essentially the western Robin Hood. However, at some point, he went insane, likely due to the realization that all of his efforts won't bring any true change in the end. Now, he's gone absolutely Ax Crazy.
Jack Krauser from the Resident Evil series qualifies as such, especially when Darkside Chronicles paints him with an initially heroic light, but after his arm was heavily injured (which resulted in him being fired from SOCOM due to it never recovering), as well as becoming increasing envious towards Leon, he eventually fell to the depiction of him in Resident Evil 4.
Vilmor, in the Dragon's Grasp arc, had been imprisoned for destroying the town of Bask and hurting the trust of great ice dragon Cryozen (which Vilmor had bonded with). Subverted big time. It turns out that SHE wasn't responsible for the destruction of Bask, Dragon Master Frostscythe was, and she was just looking for Cryozen before it died. Furthermore, Frostscythe was her childhood friend who felt that he was shortchanged by the Dragon Lord order because of his ice elf lineage; the whole Bask incident was an elaborate ploy to sever the bond of trust between dragon and Dragon Lord.Before you ask, yes.
Subverted in Touhou with Byakuren's back story. She was a revered nun in ages past, but realizing her own mortality after the death of her brother Mokuren, she dabbled with witchcraft and turned into something not human. The people eventually sealed her in the Pandemonium...but more due to her becoming more understanding to the Youkai, and not because she's turning evil. If anything, her "fallen" status made her a better person.
In Mega Man X 4, Sigma is revealed to be a victim to this trope to Zero before X1.
In the MMORPG Maple Story, Empress Cygnus in the future. Because they were weaker than the regular adventurers of the Maple World (shown by the fact that they can only go up to level 120 instead of the regular 200), Cygnus wanted to increase the power of her knights so they could match up with the rest of the world. To do so, she looked for the Tree of Life, which she found. However, it was a trap laid by the Black Mage, and combined with their crippling insecurity, the Empress and her knights were corrupted by the Black Mage, and began to destroy the Maple World.
According to the instructions manual for Super Mario Bros, Goombas were said to be former residents of the Mushroom Kingdom who betrayed Princess Peach and the Toads and sided with Bowser. Not so much in later games, where some Goombas are instead portrayed as allies.
The cast of Last Scenario is littered with heroes and wannabe-heroes who are used, deceived and broken in various ways, so naturally one of these ( Castor, the game's Big Bad) would come out of it, while the game details his descent down the slippery slope, until even his most loyal allies join the other side in an attempt to bring him back..
Whether Abysswalker Artorias is this or a Defector from Decadence is the subject of much debate in Dark Souls. Artorias was one of Lord Gwyn's four great knights, making him one of his top lieutenants. An unknown number of years ago, the Darkwraiths (Humanity devouring dark knights of the darkness) appeared. They were so dangerous that it eventually resulted in outright sacrificing a city. Artorias was charged with hunting the Darkwraiths, but instead joined them for reasons that are not known.
The DLC elaborates on Artorias' story. He did not join the Darkwraiths, but rather was rewarded for stamping them out with a blessed pendent. However, in a unrelated event in Oolacile, Artorias is defeated by Manus, the Father of the Abyss, who also appears to have single handedly destroyed most of the country in the process. As a result of this, Artorias is not only corrupted, but appears to have been driven irredeemably mad and needs to be put down at the players hands.
Gabriel Belmont in the Castlevania reboot Lords of Shadow is shaping up to be this. Despite his valiant efforts in the first game to cleanse the world of dark beings, the DLC epilogue chapters and post-credits epilogue cutscene suggest that in his time fighting the darkness, it somehow inadvertently rubbed off on him - and his proximity to it ultimately resulted in his transformation into Dracula.
Lobelia used to be a good guy a thousand years ago in Duel Savior Destiny, but due to a combination of resentment, a persecution complex, envy and genuinely believing that a world built around the strong dominating the weak would be best, she eventually turned on her companions and nearly caused the end of the current world order. In the present, her importance towards this aim has declined, but she's still working towards it as bitterly as ever.
Ulfric Stormcloak from Skyrim. Precisely where he stands on the sliding scale of anti-heroes/anti-villains is up for debate, but it's definitely lower than where he started out; a former student of the Greybeards and Imperial Legion officer, he used the training he received while studying to become a pacifist monk to commit regicide and ignite a civil war against the very empire he once served. He hashis reasons, though.
Archer in Fate/stay night. He gets to have all of the above ways of breaking him. Technically, he still believes that his ideal is correct, he just realizes that it's way bloodier than he thought it would be and would rather not exist than be forced to continue with it. He pulls off a pretty impressive Batman Gambit to do so.
Both Miko and Redcloak in The Order of the Stick. While neither of them were ever truly heroes, both were, at the bare minimum, decent people, before their Moral Event Horizon. The former actually fell, losing her paladin powers.
Miko averts this at the same time. While she falls, she never aligns herself with the bad guys (violently rejecting an offer to do so). She's trying to do what's right, but has difficulty determining what that is, largely due to her pride.
Sungod V has a good example of this trope. Subvert on the Hero part though.
Mister America, from the Global Guardians PBEM Universe, fought the Nazis, helped defeat them, and came back to a hero's welcome. He took off the costume, revealed his real name, and went to Hollywood to become an actor. And then he threw it all away by testifying before the McCarthy hearings as a friendly witness. He's since tried to make multiple comebacks, but every time he does, someone reminds him of the day he left his friends swinging in the wind before a hostile Congress.
Armsmaster is admittedly not the nicest hero in the parahumans setting. However, in the Extermination arc, he finally goes all the way overboard, endangering the truce that gives a slim chance against Endbringer attacks simply for his own career — a move which has him quietly placed under house arrest (although he is allowed to escape in order to become the superhero Defiant).
Panacea was considered one of the most selflessly heroic people in the entire Brockton Bay until the successive psychological stresses she suffered because of Tattletale, Marquis, Bonesaw, and Jack Slash made her crack and accidentally inflict total Body Horror on her sister.
In The Monster Girl Encyclopedia, the Demon Lord's army has a whole unit made up purely from this trope. These people were heroes who failed their mission to slay the Demon Lord. Men are charmed and women become succubi. The old comrades then reform their party under command of the Demon Lord. It's played for laughs, although they're probaly the most powerful fighting force in the Demon Lord's army, but the former heroes are mostly too busy screwing their heroines and are only seen in actual combat when there's a serious threat. And the Demon Lord's husband is none other than the hero who slayed the previous Demon Lord. But considering his motive, to create a world where humans and monsters can live in harmony, it's arguable if he's really fallen.
Linkara was well on his way to becoming a very tragic (for him and the rest of the world) one of these before Margaret stepped in, prompting a journey where he had it thrown in his face just what he was becoming.
The Justice Lords from Justice League, following the death of their Flash, became Knight Templars and transformed their earth into a metahuman-ruled dystopia where dissidents and supervillains were lobotomized. The Superman quote from the episode "A Better World" is given just before he crosses the line and kills Luthor, who was responsible for Flash's death, with his heat-vision.
Wasp of the same series, who, after being falsely accused of being a traitor, spends over 50 years in a prison and goes insane. Poorguy. That's not to say that he was a decent guy. As Bulkhead put it, "You may not have been a traitor, but you were never a good bot."
Wheeljack from Transformers Armada. After believing that Hot Shot abandoned him and left him for dead, he does a Face Heel Turn and joins the Decepticons. He comes back for revenge.
Depthcharge was apparently a model Maximal before Rampage wiped out Colony Omicron, turning him into a grim and obsessed hunter.
And Megatronus Prime, the thirteenth original Transformer. You should know him as the Fallen, with his original name being taken by Megatron, who in most continuities idolized him enough to take his name.
Megatron himself generally follows this in most continuities. What usually happens is, at first, the Autobot regime is corrupt and dictatorial, and Megatron rebels against that, being a genuinely heroic revolutionary fighting for social change (often for the sake of a highly oppressed working class). Then right around the time he actually manages to get rid of the 'bots responsible to the point where the Autobots have enacted the change he wanted and become a genuinely morally upstanding society, the stress and thrill start driving him to become even more evil and power-hungry than the ones he was fighting, and so the civil war he started continues with the "good" and "evil" labels having switched somewhere along the way.
Danny Phantom finds this as his future. He did some pretty disturbing things in that future, including murdering his human self along with probably hundreds of others and doing millions in property damages...at least. Quite shocking, given the otherwise childish, campy tone of the series. Danny, upon seeing this, is extremely horrified by his actions.
Teen Titans had Terra, who started off as a good-natured girl with unstable powers, but was eventually drawn to becoming Slade's apprentice in exchange for him teaching her to control her powers, leading to her betrayal and becoming a villain.
Chase Young of Xiaolin Showdown used to be a heroic monk under Grand Master Dashi, until Hannibal Roy Bean convinced him to trade his soul for an immortality potion. Since then, he's been one of the world's greatest evils.
There's also Cree Lincoln, Numbuh Five's sister and once the best operative the KND had, who later became one of their worst enemies. Also counts as a Broken Pedestal and a case of Cain and Abel in Numbuh Five's case.
Later revealed that he didn't really go to the dark side. He's a Reverse Mole and Dick, Wally, and Artemis are in on The Plan.
However Aquagirl really did die, and he really was geniunely devastated by the reveal of his true parentage, to the point where Wally isn't totally confident Aqualad won't end up becoming this for real in the end after all, especially since he's growing a genuine bond with his father, while becoming more and more detached from the team.
"Before The Dawn" reveals that Blue Beetle II, Jaime Reyes, becomes this in Bart Allen's future and apparently plays a key role in initiating the apocalypse—Jaime is the main reason Bart traveled to the past in the first place.