Or they'll be like a fusion of the second and third example and decide that killing/destroying everything is the ONLY way to save EVERYONE from the pain/pointlessness of existence, often becoming a Straw Nihilist and an Omnicidal Maniac.
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.
Not really he was blackmailed into it by the FUG. They basically are threatening to kill all of Viole's friends if he doesn't work for the FUG
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.
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.
The Big Bad of Drifters is heavily implied to be this. Well, he's actually implied to be Jesus Christ turned Omnicidal Maniac. Plus, some of the Offscouring can qualify - Joan of Arc becoming a homicidal pyromaniac after being burned at the stake, for example.
Aga Mbadi from Battle Angel Alita: Last Order. He was once a renowned hero around the solar system for hunting down a dangerous terrorist. After being forced to watch the only woman he ever loved slowly die in his arms and acquiring his insane nemesis's brain chip, he has turned into a Manipulative Bastard and Nietzsche Wannabe who considers everyone but himself lower than dirt.
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.
Suzaku from Code Geass was the poster boy for what an Eleven can become in Britannian society. When Princess Euphie dies, he turns into the White Grim Reaper.
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.
From Marvel's Golden Age, there was Thomas Halloway, the original Angel. In those times, he was a hero who fought the Nazis alongside the Sub-Mariner and the original Human Torch. But in modern times as an elderly man, he started to take morally questionable means of fighting criminals, financing and running the murderous vigilante group Scourges of the Underworld, which has assassinated a large number of lesser supervillains. (The worst part is, while he ran the group, the the Scourges ultimately reported to the Red Skull, someone all the heroes of the Angel's time opposed.) The USAgent confronts him eventually in his mini-series; the former hero is wounded and arrested, as are many Sourges, but the Angel himself was released for lack of any concrete evidence. It was assumed he resumed a quiet life. Still, the heroic Angel lives on in the form of his grandson Jason, who was given his costume during The Marvels Project limited series.
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. At the end of her own series she was forced to send to the original Supergirl back to her own reality, where she was Doomed by Canon. This left Linda broken and she promptly gave up the costume. A later series has her end up in Hell off-panel, but it's considered to be Fanon Discontinuity.
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 W.I.T.C.H. Nerissa used to be the leader of the previous generation of Guardians and the Keeper of the Heart before letting herself be corrupted and murdering Cassidy, who had replaced her as the Keeper because the Oracle had seen she was letting herself being corrupted and hoped to prevent this. Also, Will, the leader and Keeper of the current generation, fears to become this since she faced Nerissa, but managed to stay on the side of good.
While still a "hero", Rorschach from Watch Men became a murderous vigilante after failing to save a girl killed by her kidnapper.
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.
Gentaro in Horseshoes and Hand Grenades, having been placed into a More Than Mind Control situation from Ophiuchus. Ryusei also becomes this, due to Tachibana brainwashing him and converting him into a cyborg in an 'attempt' to fix all the mistakes he did.
Two-thirds of the Mane Six descends into villainy in the backstory to Fallout: Equestria, set during the war between Equestria and the zebra nation. Twilight Sparkle becomes an amoral Mad Scientist and Evilutionary Biologist obsessed with finding a way to turn normal ponies into alicorns, Rarity experiments with dark magic and creates propaganda that encourages hatred against zebrasnote She eventually has a My God, What Have I Done? moment and tries to redeem herself and her friends, but by then it's far too late., Pinkie Pie becomes a drug-addled psychopathic torturer who runs the Equestrian equivalent of the Ministry of Love, Rainbow Dash becomes a Blood KnightGeneral Ripper obsessed with her own martial prowess and kill count, and Fluttershy passes military secrets to the zebras out of a misguided sense of kindness, directly leading to Equestria's destruction.
Also Red Eye. Everything we see of his backstory points to him being an expy of the Lone Wanderer, but by the time he appears in the story, he's fully into Well-Intentioned Extremist territory and willing to do absolutely anything to save the wastes.
The Fierce Deity is stated to be this in Blood and Spirit. It is revealed that he was once the chosen hero of Termina's guardian goddess, Terminus, just as Link is to Zelda/Hylia. He fought Majora and won, only to end up being corrupted into what he is now. Terminus has Din, Nayru, and Farore send Link and Zelda to Termina to help and after Link has weakened the Fierce Deity, she and Zelda use the Song of Healing in an attempt to restore him to his former self; unfortunately, by that point, he has been far too corrupted by Majora to be saved, and is instead converted into the Fierce Deity's Mask. Though heartbroken over the loss, Terminus is nonetheless comforted by the knowledge that at least her hero is finally free from Majora's control.
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.
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. It's implied he was corrupted by the Fallen in a moment of weakness.
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.
Battra from the Godzilla Heisei series counts. He originally started out as the Earth's protector until he went too far which led to the destruction of an ancient civilization and his imprisonment by Mothra.
Erik Lehnsherr gets a more heroic treatment than usual, making him more of this trope than of the typical Anti-Villain fare.
Same for Mystique (which is actually the reverse of the comics continuity in which Mystique begins as a murdering villain but later on becomes a hero and a member of the X-Men).
Michael Corleone of the The Godfather films. He chooses to fight in World War II instead of following in his crime family's footsteps, much to the shock of his brothers. He returns home as a respected war hero, and plans to start a family with his fiance. Then his father is nearly killed by an ambitious drug runner, forcing Michael to act. He eventually gains control of the crime family and possibly becomes even more ruthless than his own father by the end of The Godfather 2.
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.
Many of the Forsaken from The Wheel of Time were this. Demandred, Sammael, and Be'lal were all great generals on the side of Light (and all three of them turned to the Shadow out of rivalry with the Light's other great general, Lews Therin Telamon); Graendal was a famed ascetic who went bad after deciding that no one could possibly measure up to her extreme moral standards; Ishamael was considered the greatest philosopher and theologian of his age, but learned one too many Things Man Was Not Meant to Know and went mad from the revelation, becoming in essence The Antichrist. Subverted with Semirhage, who though a renowned healer was always a sociopath and sadist deep down and turned to the Shadow early on upon realizing that the Dark One would let her use her talents in ways that society would never accept.
Live Action TV
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.
Willow Rosenberg of Buffy the Vampire Slayer became the Card-Carrying Villain version of this trope after witnessing the death of her girlfriend. Magic high also leads to her becoming this. Luckily, the transition was temporary in the TV series. There was also an Alternate Universe book trilogy ("Wicked Willow") that explored what would have happened if she had stayed that way.
In the Season Nine comics we learn that while she is holding it together around Buffy, Willow is hell bent on bringing magic back, believing the world is going to end and she has to save it. Faith also took the Card-Carrying Villain route after accidentally killing a human, also temporary-ish (though its acknowledged in-universe that her actions while she was a villain went too far to just be forgiven and forgotten).
Angel plays it straight with Daniel Holtz, season three's Big Bad. He was once a force for good, but Angelus and Darla destroyed his life, slaughtered his family, and reduced him to a revenge-driven monster. At his best, he is a Noble Demon, at his worst he's a petty old man willing to sacrifice the lives, happiness, and even sanity of people who love him in order to have his revenge.
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 reminiscent 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.
The War Doctor in Doctor Who becomes this due to the horrors of the Time War. As for Rassilon, he was always revered despite being a bit of a genocidal maniac, but the War pushes him off the crumbling remains of his pedestal.
The Brotherhood of Makuta from BIONICLE were a group of militaristic biologists who created all animals in the Matoran Universe to preserve natural balance, and were later assigned to govern over various regions of the Universe along with the local leaders. Due to their secretive nature and Elemental Powers of shadow, the society began worshiping Mata Nui and the Toa heroes over them, even after their armies had just saved the Universe. Thus, they grew envious, and a glitch in their A.I. caused them to turn evil, with Teridax overthrowing the old Makuta leader and becoming the story's Big Bad.
LokiLaufeyson (you know, the bad guy in Thor and The Avengers), 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, the Ragnarok.
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.
It's a bit more complicated than that. Actually, a lot more complicated than that. It was Ric Flair himself who decided that Michaels should be the one to retire him (well, not necessarily retire him, but be the one given the next shot at trying to retire him), and Michaels was very, very reluctant about taking up the challenge, since he knew that Flair couldn't win. When the newly retired Flair was given a glorious and highly emotional send-off the night after WrestleMania (where, of all people, Jericho was the most effusive in congratulating Flair), everything seemed to be all right. Until, that is, Batista began to take issue with Michaels having effectively banished his mentor from WWE; for a while, it looked as if Batista would be the one making a Face-Heel Turn. Jericho just jumped on the HBK-bashing bandwagon for no other reason than to humiliate his longtime rival (he did have quite a bit of history with Michaels, after all).
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. The idea is 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 40,000. 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 favorite 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 general, the theme of Chaos is that many think they can control it or they will not fall to it's addiction. Sadly many are horribly wrong. Freedom fighters pray to them to grant their boon, not knowing that they invite mutation and possession into their bodies. Psykers are tempted with control and normality, but instead are commandeered by daemons. Even entire chapters of space marines (most notably the traitor legions and, more recently, the Astral Claws) thought they were doing what was right in defying the Imperium and protecting innocents, only to be branded as heretics and hunted down, ironically forcing them to turn to chaos to survive.
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...
Mage: The Ascension puts this spin on its primary "antagonist" faction, the Technocratic Union. Back in the olden days, they were bona-fide heroes, fighting the old-tyme Sorcerous Overlords of the world as the Order of Reason, in the name of God, the common good, and the Muggles of the world. Nowadays, while they haven't quite turned into complete villains yet, they've certainly fallen very far from their idealistic past, and quite a lot of Technocrats are more concerned with control and stability than making a positive difference in the world. And the fact that they're much better at making people not believe in magic than they are at making them enthusiastic about science is one of many factors slowly killing the world by inches. Unlike many World of Darkness antagonists, Technocratic characters are completely playable, and their sourcebooks often stress the possibility for player characters to be Science Heroes rather than stodgy, soulless bureaucrats and to act as idealistic Internal Reformists working to make the Union a better place.
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.
By that same token, Malygos the Spell-weaver, the Aspect of the Blue dragonflight. Best remembered from the pre-Sundering days for his playfulness and good humor, being the protector of magic. After Deathwing wipes out most of the blue dragons and seriously wounds Malygos, the latter spends 10,000 years in isolation, going mad from loneliness and betrayal. Despite snapping out of his madness in Day of the Dragon to help fight Deathwing and free Alexstrasza and the rebirth of the Blue dragonflight, Malygos never goes back to his cheery old self. Instead, he declares war against all non-dragon magic users, forcing Alexstrasza to assist heroes in killing a fellow Aspect.
Sarah Kerrigan in Starcraft. Though she didn't "fall" so much as "was thrown, had her sense of morality suppressed", 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 suppressed, 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.
Pre-release info for the third game implied the same would happen to the heroes of the second game (except the Barbarian), but it was ultimately scrapped.
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 was originally a quirky, cheerful, affectionate, paternal sort of man (though he still was a supreme Badass not to be trifled with), who ends up going through a major Break the Badass routine in Snake Eater, Portable Ops, Peace Walker, Ground Zeroes and The Phantom Pain. 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), as well as with Ocelot and EVA and creating Outer Heaven and Zanzibarland to plunge the world into eternal war and seek revenge against those that crushed his dreams and ruined his life, before meeting his end at the hands of his "son"/clone, Solid Snake, which was prophesized by The Sorrow, Elisa and hinted at by Paz.
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.
Major Zero as mentioned above. To elaborate, he was directly (albeit, unintentionally) responsible for many of the things that happened in the series by creating the Patriot AIs, but he genuinely meant well before things went horribly wrong. And by the time the AIs went rogue, it was too late for him to realize and fix his mistake. And when Guns of the Patriots rolls around, he's fallen from grace in more ways than one, being revealed to be reduced to a very old man confined to a wheelchair, on life support and suffering from severe dementia. Big Boss pulls the plug on him, and as he dies, the audience is treated to flashbacks of the Major in his younger days, emphasizing that he was a good man who went about things the absolute wrong way. Alas, Poor Villain indeed.
The 7 Heroes of Romancing SaGa 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 they're forced to work with a terrorist group.
And Shepard is definitely seen as this at the beginning of Mass Effect 3 given the terrorist connection and that they was forced to kill over 300,000 people to slow down the Reaper invasion. One news report early in the game even refers to them 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 tests 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.
Gabriel Belmont in the series 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.
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 overthrowing the previous viscount of Kirkwall, Perrin Threnhold, when he tried to expel the templars from the city. While not everyone looks upon the templars fondly, Perrin Threnhold is generally regarded as a tyrant and robber baron who brought about his own demise. In the end, Meredith's paranoia over blood magic drives her utterly mad and she tries to use an Artifact of Doom to kill Hawke.
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 X4, Sigma is revealed to be a victim to this trope to Zero before X1.
In the MMORPG MapleStory, 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.
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.
In Disney Princess Enchanted Journey, the Big Bad, Zara, is an ex-princess who refused to learn princess virtues and was banished from her kingdom as a result. She came back with evil powers and sought to ruin worlds of other princesses and stop girls from becoming princesses.
While not the main character of Ravenmark: Scourge of Estellion, Livia Cassianus is, nonetheless, an important character, being the protagonist's Love Interest and a member of the Court of Shadows, the Imperial House's highly-trained spies and assassins (all Heroic Bastards of said House). Livia is a good-hearted, noble person. This is why the late Emperor Sergius Corvius has been secretly grooming her as his successor over his own legitimate children (his eldest daughter doesn't want the job anyway). After Grecian, Sergius's son, is overthrown by La Résistance, Livia is crowned Empress Livia Corvius and personally leads the Imperial Mark against the invading Kaysani hordes. But when the Big Bad Alejo de Porres chooses to suicide-bomb himself to kill Livia, her lover Calius Septim, the main character, sacrifices himself to keep her safe. Livia becomes determined to punish all responsible. Ravenmark: Mercenaries takes place 6 years later. By this time, Livia is known as the Scarlet Empress for her brutal policies and unwavering desire to crush all enemies of the Empire, both domestic and foreign. In large part, she is responsible for Estellion becoming a Vestigial Empire, barely strong enough to fight the newly-arisen Varishah Federation and the formerly-allied Commonwealth of Esotre in a three-way stalemate. Additionally, one of the most important cities in the Empire has seceded and remains a haven for mercenaries.
Cody Travers from Final Fight and later the Street Fighter series. While it is not actually seen in the games, multiple games tell the story of his downfall, which occur after the ending of the original Final Fight. Cody and his friends go out to save his girlfriend from the Big Bad in Final Fight. On the way, he beats up a corrupt cop named Edi, who later arrests the hero for assault in battery. Next, his girlfriend dumps him, and leaves the country to study abroad. Afterwards, he is let out of jail and tries to get revenge by fighting criminals outside. He gets arrested again, and becomes addicted to fighting within prison. He then eventually breaks out, and joins the Street Fighting cast in their tournament(s). After all these events, he usually claims that he will never be the hero again, and often states that all he has left is fighting (which he often exclaims is pointless).
Izbel from Tears To Tiara 2 is an interesting case. She does a Face-Heel Turn but she was ordered to do so as the final order of her commander and the man she loves. And the entire string of events, and really her entire life, has caused her to loose faith in people.
Captain Walker in Spec Ops: The Line. He originally came to Dubai to help the stranded American regiment there. By the end of the game, however, his obsession with being a hero has caused him to massacre the very people he was supposed to save, horrifically slaughter dozens of refugees with White Phospherous, condemned the remaining refugees to a slow, agonizing doom via dehydration, and lead his comrades to their entirely preventable deaths.
Green in Gunstar Heroes, was the strongest of the Heroes, but betrays Red, Blue, and his sister Yellow by deciding to join the villains plan to revive the evil god Gold. He does a Heel-Face Turn in the end and sacrifices himself to stop Gold.
Borderlands: The Pre-Sequel!, a Mid Quel for the two main series games, reveals that Borderlands 2's Big Bad Handsome Jack was one of these. In the Pre-Sequel, Jack is shown as a noble and good-hearted man (though still ambitious and bloodthirsty) who's willing to step up and fight and helped save Pandora's moon Elpis. However, a series of devastating events including several betrayals as well as having a repository of alien knowledge downloaded into his brain fractured his mind and turned him into the egotisticalDirty Coward and Comedic Sociopath seen in the second game. It also reveals the origin of his iconic mask and his "Handsome" nickname: his face was scarred by Lilith, which also explains why he's fixated on killing the Vault Hunters and everyone they ever cared about. However, the Pre-Sequel's narrator, Athena, feels that Jack, the Hero of Elpis died and Handsome Jack is nothing but a grotesque mockery of everything he once stood for.
Tatsumaru from Tenchu 2 only turned to evil after a case of amnesia. He got his memory back but chose to fight for the bad guys out of guilt (and he's in love).
Archer in Fate/stay night. He gets to have all of the above ways of breaking him; in fact, contrary to the page quote, he died a hero and still saw himself become the villain. 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.
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.
Shayera Hol, AkA Hawkgirl was considered this after it was discovered she was The Mole during the Thanagarian Invasion, which caused her to leave the League, though Alfred begged to differ. However, upon rejoining the Justice League, she became a hero again, but discarded the name "Hawkgirl", due to the lies, disgrace, and treachery that the name carried. Therefore as of Unlimited, Shayera considers her alter ego to be a fallen hero.
Two-Face again, this time in Batman: The Animated Series. Before his transformation, Harvey Dent was a regular character on the show— an ally of Gordon, and Bruce Wayne's closest friend. In the series' final episode, Dent became an Anti-Villain known as The Judge, and actually tried to kill The Penguin, Killer Croc, and various other hardened criminals...including himself (of which his two previous personalities were unaware). Thus did he (somewhat) earn his redemption.
Ethan Bennett was a similar character on The Batman. He was an honest cop and a close friend of Bruce until after already under stress due to an argument with Chief Rojas, he became the victim of Mind Rape by The Joker'', and was then exposed to the villain's Joker Putty, turning him into a clay-like monster. The whole experience drove him mad, and turned him into this continuity's version of Clayface, who was more than capable of murder.
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.
In fact, the KND seems to have a problem with traitors a lot. That may be the whole reason they inflict Laser-Guided Amnesia on operatives when they retire. On a positive note, both Chad and Maurice were Fake Defectors, subversions of this Trope, although in Chad's case, the revelation didn't make Numbuh One like him much.
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.
Subverted with Finn who is turning into this in later seasons after loosing both Princess Bubblegum and Flame Princess and finding out that his Disappeared Dad has become a bumbling old fool. But aside from all that, he's staying on the path of good.
Billy is a fallen hero by the time Finn and Jake find him, hiding passively in his cave.
Billy: All my life I've beaten on evil creatures, but new evil keep popping up! Kicking their butts was a hopeless effort!