In Batman: Arkham Asylum and Batman: Arkham City, Poison Ivy can be classified as a Woobie, Destroyer of Worlds because she is an eco-terrorist essentially because society harms Nature, her "babies", and she wants humans to stop hurting the planet. In addition, in Arkham City she locks herself up and really just wants to be left alone when Catwoman comes in and she sees herself forced to kill her.
Kyrie from Sands of Destruction is a unique example of this trope in that he is more than capable of destroying the world (it was what he was created by the world itself to do, after all), but he doesn't want to do this in the least! His power brings about many a Tear Jerker in the game.
Fortune and Vamp from Metal Gear Solid 2 (with a little more emphasis on the Woobie bit in the former case and the Destroyer bit in the latter). The former endured the deaths of everyone dear to her, and as such is a Resenter and Death Seeker who happens to cause a lot of carnage with her BFG. The latter was the victim of terrorism, having to resort to cannibalism in order to survive in the ruins and developing a troubling immortality in the process.
Duminuss from Super Robot Wars Reversal, an artificial being (not sure of what she actually is, as she is only seen as a trippy eye glyph with a feminine/shota voice... and several Humongous Mechas) whose only wish is to know her purpose. Her creator shunned her, and then she killed it. Actually, her creator, Dark Brain, didn't die. He just implanted that memory into her for the lulz and left her. She shifts dimensions and invades the EXCELLENCE team labs searching for a time machine, to ask her creator for her purpose. She constructs 3 children, who are loyal and fight for her. Then Duminuss is destroyed, and her children kill themselves to bring her back. Then the heroes kill her again. She explodes, crying over how she'll die without ever knowing what was her true purpose. Unfortunately, Original Generation Gaiden threw this out of the window and made her an unrepentant Jerk Ass...
It's hinted that this isn't the same Duminuss, and Dark Brain created multiple ones to do his dirty work. If it wasn't messed up like R's was, it makes sense that it's not the same
Sirus, aka Dark Emperor Griffon, from Dark Cloud 2. Originally a member of the Moon Tribe (aka anthropomorphic bunny) who loved nothing more than the flowers in the palace gardens, he was accused of trespassing. Alexandra interceded for him and made him the Garden Keeper. But then invading armies searching for the Atlamillia utterly annihilated the kingdom, leaving it a blasted wasteland, and killed Alexandra. His grief was so great that he swore vengeance on all of mankind, and started systematically erasing it from existence via Time Travel, acquiring the MacGuffin for himself so he could reduce the world to nothingness. Regardless, the player and the protagonists are made to feel sorry for him by means of flashback scenes scattered throughout Moonflower Palace.
Yomiel in Ghost Trick. He was falsely accused of giving secrets to the enemy and scared into thinking that he had no hope of acquittal, so he stole a police officer's gun and broke out of the police station, taking a little girl hostage out of panic. A shard of the just-crashed Temsik meteorite penetrated his back, freezing his body at the moment of death and severing it from his soul. By the time he finally pulls himself together and returns home, his fiancée has killed herself because he's been officially reported as dead. Yomiel is forced to wander the world alone, unable to die but not truly alive, separated from the rest of humanity. The isolation nurtures a darkness in his heart, making him want revenge on those who put him in that position, but he still desires most to have some way to lead a human life. His only friend and companion over those ten long years was a cat... and while trying to manipulate Lynne into shooting his shell, Yomiel accidentally kills him. Even his victims feel sorry for him when they find out his story.
F.E.A.R.'s Alma is a dead straight example of this trope. Having been driven insane by her own psychic powers as a child, experimented on and locked up since she was eight years old, medicated into a coma and locked away in a shield vault for most of her life, forcibly impregnated and then having both of her children taken away, then killed once the project was terminated, all by her own father, and then repeatedly shot at by one of her own children while trying to embrace him, it's no surprise that the second she gets loose, people die. F.E.A.R. 2 continues her rampage as she tries to get revenge on everyone who ruined her life, and kills anyone who happens to get in her way.
He's built up much more into this in the prequel, Crisis Core, where his two best friends turn out to be flawed prototypes, abandon Shinra, and are hunted down like animals. One goes insane and the other commits Suicide by Cop in fear of doing the same, and in a few months, Sephiroth loses the only people he could relate to and any pretense of trusting the organization that rules his life. So when Genesis and Hojo set him up for The Revealabout himself and Jenova starts messing with his head, he cracks because he's got nothing worth holding on for. Also, Crisis Core shows that prior to the Nibelheim incident, Sephiroth was a fairly decent and caring person.
A much more straight example of this trope is Dyne.
While Ultimecia may not be a future version of Rinoa, her own backstory is just as tragic. Due to being a Sorceress, she was persecuted her entire life because of the terrible actions of evil sorceresses in the past. Not only that, but thanks to history, she knows she is destined to die at the hands of a bunch of teenage mercenaries and all her plans are based on a desperate desire to escape her fate. Sadly, her actions in the past to achieve this goal caused the start of the persecution of Sorceresses in her own time. The story itself screws her over because of the subtle method of storytelling in the game, her backstory has to be pieced together from hints and comments by characters and plot events, making her seem like a Giant Space Flea from Nowhere.
Shuyin in the sequel fits, however. Given that he was subjected to a thousand years of non-stopMind Rape until the start of the game, it's kinda understandable...
It's implied that this affects Kefka Palazzo in Dissidia: Final Fantasy during his surprisingly touching final scene in which it is implied that the reason Kefka became an Ax-CrazyNietzsche Wannabe was because he's so damn insane, he thinks there's nothing worth living for except destruction. Terra herself says that he was destroying to try and fill his "broken heart".
Speaking of Dissidia, Chaos becomes one of these at the end. After destroying Cosmos, he realizes that he has no equal anymore (and possibly ends up horrified to remember that Cosmos was actually his artificial mother), and that the cycle of conflict is coming to a close. So he decides to destroy everything first.
While you don't find out exactly why Garland betrayed Cornelia, his actions in Dissidia are said to be motivated by him pitying Chaos and Cid of the Lufaine. Also, One Man's Monologue depicts him lying paralyzed in the destroyed parallel world for several days and clearly shows that he felt bad about his time loop shenanigans in hindsight.
Caius Ballad. He was tasked by Etro to forever guard over the Seeress of Paddra (Yeul), and 'blessed' with Etro's own heart, making him immortal. Which means he's had literally millennia of the same cycle; Yeul dies young, is reborn, dies again... etc. By the time the game starts, his mind has become so warped by the pain of her thousands of deaths, that he believes Yeul's cruel existence must be put to an end, and that stopping time itself from existing (after all, how can a seeress have visions if there's nothing to see?) is the only way to truly save her.
Sephiran/Lehran tries to call the judgment of a goddess down upon Tellius in Fire Emblem: Radiant Dawn, all because he thought that it would be the only way for him to finally be put out of his misery and that humanity was completely irredeemable after witnessing 850 years of slavery, tragedy, and war.
Zephiel from Fire Emblem 6 and 7was a talented youth who did his best to win approval from his father, the king of Bern. But the harder he strove, the more distant his talentless father grew, and the fact that he was born from a loveless marriage didn't help either. The final straw came when the king poisoned his heir's drink, several years after a first assassination attempt failed. Zephiel's closest retainer, Murdoch, came up with the idea of faking his death to get him out of there. However, the king opened the casket, causing Zephiel to finally snap and stab him. According to his half-sister, Guinevere, Zephiel never smiled again. Years later, he (now king of Bern) was stirring up quite a bit of trouble in neighbouring countries, trying to offer the land back to its rightful owners because Humans Are Bastards. Needless to say, he had to be killed... with his crestfallen sister's help, no less.
Another example is the true Big Bad of the Tellius games, Prime Minister Sephiran. He was once a great hero, one of four champions of Ashera, Goddess of Order, in her war against Yune, Goddess of Chaos. He was a kind man, and a member of the most peaceful race on the continent, the heron laguz; indeed, once Yune was subdued, he defied Ashera's order to kill her and simply sealed her away. As Ashera thereafter proceeded to thereafter seal herself, he promised her that Tellius would see no war for the next thousand years, with the goddess adding ominously that if that promise were broken, she would destroy the world. Sephiran married a fellow hero and started a family, a family that would found the greatest country on the continent, which, given his absurdly long lifespan as a heron, he would continue to serve in high positions all his life. Everything appeared to be going fantastic for him. Then, nineteen years before the main story a Smug Snake subordinate of his assassinated the country's beloved empress and pinned the crime on the herons, all but four of whom were wiped out in the resulting genocidal war. Sephiran resolved that a world in which that could happen simply didn't deserve to live, and resolved that he would create a war so devastating it would wake the goddess and bring doom to all life.
Durandal, an AI from the Marathon trilogy, was deliberately threatened by his creator in order to drive him to Rampancy (as part of an attempt to safely study the process), made to open and close doors for hundreds of years in order to stifle his creative development and slow his Rampancy, and was probably about to be experimented on more when he entered the "anger" stage of rampancy, secretly contacted hostile aliens and drew them to Tau Ceti to enslave or kill every single human on the colony or in the ship. Though he becomes less of a woobie later, when he turns into a Bad AssChessmaster.
Let's be fair to the bastard. He brought the Pfhor as a distraction so he could get loose. Once that was done, he started working on stopping them, freeing their slaves (admittedly, to work for him), and helping the Security Officer do that which he does so well. The extermination of those on Tau Ceti IV was not intentional.
The King of Planet FM from Mega Man Star Force. Everybody, including his family, wanted to kill him to overtake his throne. As a result, he stopped trusting people. He destroyed Planet AM and almost Earth, because he thought that the people there would want to kill him as well.
Jack and Queentia from Star Force 3 also fit this trope like a glove. They were once the prince and princess of a small, but prosperous country, which was attacked by neighbor nations for their advanced EM technology. And it just went downhill from there...
You could probably also say this for Burai/Rogue, also in the 3rd game. After the first time you fight him on your way to fight Jack Corvus, he may have shared his backstory, saying something along the lines of "Go ahead and save him. Later on, he will betray you."
Shin Megami Tensei IV introduces the White, who are quite literally the Anthropomorphic Personification of Humanity's despair due. They are quite tired of the endless war of Order Versus Chaos as they believe it all plays into God's hands and only want everything to go back to eternal nonexistence. So they have this giant Magical Particle Accelerator built and a group of heroes suitably traumatized in hopes one of them will just go off the deep end, and as a living creature, give the universe a final death by breaking their machine and unleashing a chain of black holes to end it all.
Several characters from League of Legends fit this trope.
Veigar: A member of a notably short and cheerful race, he was driven insane from isolation while imprisoned in Noxus. He then spent years learning dark magic, and vowed to end conflict by bringing all nations to their knees.
Varus: The guardian of the pit of corruption whose village was burned and family was killed by invading Noxian forces. Overwhelmed by regret and fury he absorbed the power of the pit of corruption, failing the sole task he was given in the process, in an attempt to gain the power he needed to exact his revenge.
"Beware a man with nothing to lose"
While Noxus seems prone to create this sort of character, they aren't without their own woobie.Urgot was once a proud, fearsome soldier of Noxus who threw himself into the fray until the day his hands were too mangled to hold a weapon. He was awarded for his loyalty by having his hands replaced by blades and given the role of High Executioner. While following a group of soldiers on a mission, he was going to get the opportunity to kill Jarvan IV, the prince of rival nation Demacia but before he was given the chance he was chopped down by a Demacian soldier named Garen Crownguard. Robbed of the grandest moment of his life, Urgot's sheer anger and hatred was the only thing that made him survive being rebuilt as the necromantic cyborg he is today.◊
"Existance is torment."
The Shadow Hearts series has a few, but Masaji Kato from Covenant takes the cake. Having the woman you love being executed for treason? Bad, really bad. Managing to clone her, doing your best to make her clone remember everything so that you can finally be happy together, only to have to kill her again, and this time permanently, just as she starts to love you too? OUCH. No wonder he snapped after this and tried to create a new world by destroying the current one... Even the protagonists feel sympathy for him as the final battle starts.
Alessa Gillespie from Silent Hill is another textbook example. She was burned to the point of near death but kept alive in excruciating pain, force-fed experimental hallucinogenics, and forcibly impregnated with God. This was all done by her mother.
Claudia from the third game and Walter from the fourth fall under Put Them All Out of My Misery, however, since they wanted to rid the world of pain and suffering in general.
The Ur-Quan of Star Control. After spending thousands of years psychically enslaved by evil toads who force them to exterminate whole species of their friends, and finally clearing their minds only long enough to revolt by putting themselves through unspeakable agony, anyone would be in a bad mood. The nice ones want to forcibly subjugate all sentient life in the galaxy. The rest want to eliminate it altogether.
The main antagonist in Super Paper Mario, Count Bleck, who wanted to use the Chaos Heart to undo and redo all reality because he was heartbroken by his one true love. Except it actually turns out that the "redo" part is a lie. He's that messed up by the loss of his love.
Depending on how charitable you feel, the darker Forsaken from World of Warcraft fall into this trope: their penchant for obscenely lethal plagues, doomsday weapons, and tendency to respond to any threat violently are a direct result of having once been decent, devout humans and elves before having been infected by the Plague of Undeath, killed, resurrected into undeath, corrupted by the Lich King and forced to massacre friends and family, and finally breaking free of his control only to be rejected by their faith and persecuted and hunted down by any remaining friends, family, and acquaintances. No wonder so many of them snap with apocalyptic fury.
Sargeras, the creator of the Burning Legion, was so traumatized by the evil of some of the demons he fought against as the pantheon's chosen warrior that he decided that an universe where such things were allowed to happen was flawed, its attempts at Order pointless, and should be remade.
Despite the fact that he wants to destroy the world because he doesn't like how mortals are using magic, Malygos can count because, let's face it, his life sucked before he ultimately snapped. He was betrayed by his best friend, Neltharion (aka Deathwing, who had been corrupted by the Old Gods), who then went on to wipe out almost all the other blue dragons, coming very, very close to making Malygos the Last of His Kind. He later supposedly regained his sanity (after being exposed to some volatile magical energies from another planet).
Maly's prime consort Sindragosa could also qualify. One of the blue dragons murdered by Deathwing, she wasn't only enraged by his betrayal, but she assumed ALL Dragons (including her mate) had betrayed her when they wouldn't answer her cries for help returning to Dragonblight. She died very emotionally damaged. Mix in some Scourge corruption when the Lich King resurrected her to be his, umm... Dragon and she's angry enough to help extinguish all life on Azeroth.
Nessiah of Yggdra Unionhas spent the past thousand-odd years living in misery, unable to age or die, as a punishment for being a pacifist in Asgard's time of war. He has spent his life since then trying to get revenge or just free himself, and makes a nice mess of the mortal world he lives in, in doing so.
King Valentine in Odin Sphere throws a Suicidal Cosmic Temper Tantrum in the final book by using the Cauldron to turn Leventhan into a really pissed-off Sheng Long, which ends up destroying him along with the rest of existence. Granted, he got broken pretty hard before and during the story, beginning with being forced to kill his own daughter because she had an affair with the king of an enemy country, then dying horribly, along with most of his kingdom, after being betrayed by his own son, enduring endless torture in the netherworld, and escaping it only to be spitefully denied the complete destruction he was so desperately seeking, by the dude who started the whole thing by shagging his daughter, no less.
In Xenogears, we don't know for sure how many times Fei and Elly reincarnated Themselves, but for 10,000 years, the scenario has been mostly the same: they find each other, fall in love, and when they seem to be about to have a little marital bliss, they die a horrible and painful death. If you had the painful experiment he was subject to in his childhood, Fei end up with a multiple personality disorder, with TWO of his personalities wanting to destroy the world: one is able to exist independently and jump from body to body, and the other one is a Person of Mass Destruction. And that's not all: Krelian, a friend of Fei in a previous life, is another woobie ready to destroy the world if it allows him to be "reunited with God". With that many Physical Gods and Magnificent Bastards on the same planet, you can guess that the Xenogears world is not the most pleasant place to be.
Elpizo from the Mega Man Zero series exhibits traits of this trope, being sentenced to death for discovering records about a past catastrophe in the ruined library he was ordered to examine. He escapes this fate, only to get lots of people killed while leading a failed assault on his former rulers; this drives him to obsession and megalomania, and he decides that he wants to re-enact the aforementioned catastrophe.
Sarah Kerrigan from StarCraft killed her mom (and a whole mess of other folks) by way of a psychic accident, watched a kitten die of cancer, was forced to choose between killing her mentally ill father or her sadistic headmaster (she just broke his gun) and decapitate a rebel leader (and steal his head), was experimented on, and was betrayed by her father figure. Then she got infested by the Zerg. Is it any wonder that she's a little crazy?
Ratchet, of the Ratchet and Clank series - he's a walking class 1 at the very least, though he manages to avoid wallowing in his existential angst and/or loneliness pretty well by keeping busy.
Alister Azimuth fits this even better, being effectively an older, more cynical, and more ruthless Ratchet. To the point that he very nearly destroys time itself in a misguided attempt to correct his own mistakes.
Ballos from Cave Story, who destroyed the very kingdom whose people he loved and helped out after being subjected to Cold-Blooded Torture by the king (and if the Wii version of his speech is of any indication, he may have brought the torture upon himself by recklessly allowing his power to grow), forcing Jenka to seal him within the floating island. In fact, when you reach him at the end of the Bonus Level Of Hell, he begs you to kill him... or he shall kill YOU!
If there were any worthier candidate for the epitome of this trope, it would have to be BlazBlue's Ragna the Bloodedge. Not only did he lose his home at the hands of Terumi, but he was also betrayed by his brother, Jin, who cut off his arm simply because Ragna didn't pay attention to him enough. In addition, his younger sister had been kidnapped, and he was left to die. Then, Rachel saves him from death by turning him into a half-vampire, causing him immense trauma and making his hair turn white. Then, later, we find out that his sister is the template for a series of robotic clones, two of which are playable characters in the game. One of them, Noel, is a grown-up Saya for the most part, while the other, Nu, is a Yandere who wants to fuse with Ragna to complete herself and form the Black Beast, which turned the world into a crapsack one already. He's already flat-out stated that he hates everything because of these events.
Jin's desire to kill Ragna (and thus the cutting off of his arm) is due to being the World's Antibody and part of his function as the Power of Order. He is meant to be the opposite to Ragna, who is the Destroyer of the World (by being the Black Beast). However, Ragna could instead become the Protector of the Azure.
Noel Vermillion eventually becomes one. She was nothing more than a clone of Ragna and Jin's dead/missing little sister, Saya. Since she looks like Saya, whom Jin despises, he's cruel to her. Noel understandably doesn't know why. She's ridiculously sensitive about her small breasts, and gets upset whenever anyone mistakes her as a boy (which is stupid in and of itself, since she's still visibly got breasts, hooray sexism). Making matters worse is that her best friend, Tsubaki, has been ordered to kill her as of the end of Calamity Trigger. Oh, and then Terumi gets ahold of her, mind-rapes her, and turns her into an unholy implement of destruction. All the hate and rage she kept pent up has now been amplified and directed towards the world itself.
Aribeth from Neverwinter Nights. She starts as a heroic paladin and your main ally, beloved by all and considered a national hero. Then the people of the city you and her worked your asses off to save force the government to execute her innocent lover for being an unwilling pawn in Desther's plans. And the government had no say in it, the townspeople formed a mob and forced Fenthick's execution. In the next chapter, Aribeth is so filled with despair that the Big Bad is able to manipulate it into hatred and rage, turning her against the titular city (and the player). Aribeth is especially pitiful when you face her in the finale, as, with her lover dead and feeling of betrayed by Neverwinter, you can tell that she feels she has nothing left to lose.
In Neverwinter Nights 2: Mask of the Betrayer, there's Akachi the Betrayer, who, as punishment for defying the gods by trying to rescue his beloved's soul from the Wall of the Faithless, is turned into the Spirit-Eater curse, an Eldritch Abomination driven only by hunger and the instinct to possess bodies and devour spirits, always eating but never satiated.
Also in this category? The King of Shadows himself, the Big Bad of the Original Campaign. He started out as the greatest hero of the ancient realm of Illefarn. Then he volunteered for a horrifically painful ritual that turned him into a construct of pure magic, the Guardian, to make an effective deterrent to Netheril. Then the Netherese wizard Karsus tried to usurp Mystryl's place as god of magic and all hell broke loose. The Weave was interrupted and the Guardian faced destruction. He chose to continue his vigil over Illefarn by drawing power from the Shadow Weave. That's when he became the King of Shadows. Illefarn tried to destroy him and only succeeded in binding him outside the Material Plane.
Morinth of Mass Effect 2 claims to be this, stating that she never wished to be born an Ardat-Yakshi (the Asari equivalent of a succubus). Possibly subverted if it was ultimately her choice to screw and kill the galaxy's population, in that order, as opposed to her being controlled by an addiction that she never asked for in the first place.
The Overlord DLC adds a new one. Simply naming who it is would be a spoiler, but anyone who's played it will know which character it is.
Suikoden II: Luca Blight's behaviour is a lot more understandable when you've absorbed the fact that he became that way after observing his mother being murdered while his father stood by, cowering in fear.
More specifically, his mother was raped but not murdered as he was forced to watch, by mercenaries hired by Muse, the city-state his country was at war with, all the while expecting his father to ride in and saved the day, only to learn later that his father spent the entire time sitting on his throne, dithering. As an added bonus, his sister, Jillia, was the product of that rape, whom he loves and yet serves as a constant reminder of that day.
Suikoden III had this with Luc. In the first 2 games, he's a moody jerk who doesn't seem to have much motivation in participating in wars as his Parental Substitute more or less drops him off. 15 years later, he concocts a plan to destroy his True Wind Rune in a major case of crossing the Moral Event Horizon. The reason? Turns, out he was a defective clone of another rune bearer who wanted vessels to collect all 27 of them. Even worse is that his rune gives him visions of an apocalyptic wasteland. He thinks that if he destroys the rune, he'll effectively kill god and change fate. This has caused many fans to categorize him as a Well-Intentioned Extremist.
In the "worst" ending of BioShock 2, Eleanor Lamb turns into one of these. She was almost laser-guided by her mother to become some kind of Utopia, although some fans thought that she would become an Eldritch Abomination or an otherwise unpleasant form of Life that would destroy Rapture and the Topside World. In the Bad Ending, Eleanor is influenced by Delta's murders, kills her mother in revenge, and, depending on how many of the NPCs and Little Sisters the player killed, will be allowed by Delta to absorb his mind and, we assume, his powers, or will do the same, only against Delta's will. Whether the case, she swears to punish the world, while watching a storm fall over the corpse-filled ocean.
"There is no Name for what I am. But, with your help, they will never see me coming... (Fade to Black)"
In Bioshock Infinite, Elizabeth becomes this in a Bad Future where Booker fails to save her and Comstock successfully molds her into his heir. In the end, it wasn't the mental and physical conditioning that broke her. It was the complete loss of hope and the belief that Booker betrayed her that drove her into wishing to burn the world. However, her final act is to bring Booker into this world to pass on an important clue to her past self that would change things.
The King of Sorrow from Klonoa fits this trope perfectly. Long story short, he ends up being ignored by the entire world for representing an emotion that everyone hates. He goes mad and, as an act of vengeance, decides to unleash sorrow all across Lunatea, which, judging by the reactions, will cause quite a bit of destruction.
He outright says that he plans to destroy the world when you reach him in the Terminus of Tears and it's pretty clear that he's gone Ax-Crazy.
Melissa Bergman/MB in Metroid: Other M.Her only crime initially was occasionally disagreeing with the other scientists on the Bottle Ship, but for that, they decided that she had to have her emotions removed from her. She saw this as betrayal by her beloved mother figure, and as a result, everyone got horribly mauled by monsters.
The 501st legion from Star Wars Battlefront. Even though they're the Emperor's elite troops, somehow, you can't help feeling sorry for the narrator (who's quite obviously a Shell-Shocked Veteran), even when you're gunning down Rebels on Yavin 4.
Soma Cruz from Castlevania: Dawn of Sorrow - kill Mina, and he is going to kill you in revenge and proceed to become this, requiring Julius Belmont, Yoko Belnades, and Alucard to stop him.
This happened at least TWICE. One lead him to become Dracula in the first place, he was a devout man until she got killed, and went hog wild the help of Death and the stolen power of another Vampire. After a while of this, he mellowed out and met a woman who might have even redeemed him... who then got killed by a bunch of witch hunters, causing him to REALLY lose it (Aulcard came from the second pairing, and takes it on himself to help shut down Daddy when he awakens).
You know that Future Luke? Turns out, he's not Luke, but Clive. He lost his parents thanks to a time-travel incident gone wrong, which interestingly also killed Professor Layton's girlfriend, and the person responsible is now the Prime Minster! Thanks to a nice old lady's fortune, he decides to build a Humongous Mecha to level London and rebuild it. Interestingly, Latyon actually prevented him as a kid from going back into the burning building on the day of the incident and, while he doesn't know why, got the Professor involved knowing full well he'd ruin his plans. As if he wasn't a fangirl magnet already...
Anders in Dragon Age II becomes this by the end of the game. After a time growing up in the Mages Circle—a life stuck in a tower, bound to do whatever the Chantry asked him to—he escaped from the Templars... seven times. On the last time, he joined the Grey Wardens to escape more permanently. It's all downhill for him after that, unfortunately. The Grey Wardens consider him a wuss and mock him enough that he leaves, and then he lets a wayward Spirit of Justice—once a friend of his—into his body. All of this isn't too bad, but it starts getting nasty when he goes to Kirkwall. The sheer dark magic of the place corrupts Justice into a Demon of Vengeance. By the time Dragon Age II begins, he's constantly fighting for control over the influence of Vengeance/Justice. After all of this, his brooding is pretty justified. (He gets added points for being the constantly-hunted leader of a Mages' Rights group.) In the final act, though, he can't fight Justice off anymore, and essentially performs a terrorist attack on the local branch of the Chantry. Talk about a Trauma Conga Line. Ultimately, his fate is left up to Hawke.
Oersted from Live A Live, Knight Chapter. His story begins when he wins a tournament to gain a princess' hand in marriage and, in doing so, earns the accolades of the people, only for her to be kidnapped by the Demon King the following night. What seems to be a standard Save The Princess plot is soon turned on its head as the hero, Hash, who last slew the Demon King, is killed when fighting against it with the rest of Oersted's party. Then, the Demon King seems to assault Oersted in the night, only for it to turn out to be the king, who had been made to look like it—a fact discovered only after Oersted kills him. Now treated as a demon by the townspeople, Oersted returns to the Demon King's castle to save the princess, the one person who might still believe in him, only to encounter his best friend, Straybow, who seemed to have died in battle with the Demon King along with Hash. Turns out, he had orchestrated everything to make Oersted an outcast out of jealousy of the latter's success. On top of that, after Oersted battles and kills his old friend, the princess appears, accuses Oersted for not trying to rescue her when he had been trying to do so all along, professes her love for Straybow, and kills herself. With no one in the world now who doesn't loathe him, Oersted snaps and declares that, if the people want to think of him as a demon, why, then that's exactly what he'll become... the Demon King Odio. He then proceeds to slaughter every last person in the kingdom and send several incarnations of himself across time to test the virtues of humanity, thus starting the game's events.
As of Portal 2, GLaDOS becomes this. After all, she was originally a human, Caroline, forcibly uploaded into a mechanical shell, and, because she was (understandably) resistant to the orders of the people who did this, was also subjected to Mind Rape due to the cores they forced onto her, hearing voices all her life, and soon killed every living thing in the facility.
Wheatley becomes this. At first, he's your friend and guide, helping you escape Aperture, but when he connects to GLaDOS's body and becomes in control of the facility, he goes mad with power and betrays you. But during the final battle with him, if you listen long enough, he begins ranting. He hysterically claims that you've been using him and that you plan on running off with GLaDOS once you could escape. He even accuses that you didn't catch him when he disconnected from his management rail note Which is unfortunately true..., which he honestly believed would kill him! And worst of all... he's crying...
In Disgaea 3's "Human World Ending", after each of his party members dies one by one, Mao finally loses it when Super Hero Aurum kills Raspberyl.. Afterwhich, he effortless annihilates him and we are told he destroys the universe, and Mao is left all alone, floating in nothingness for all eternity.
Many of the ghosts from the Fatal Frame series have really sad backstories, and the only real reason they're attacking people is because they've been driven violently insane by the Malice/Darkness/Curse/etc that came out of the story's resident Hell Gate.
Interestingly, you hear about this from his grandfather, who was well aware of the Parental Neglect and regrets not helping him, implying that it's possible Cyrus could have turned out normal if his grandfather had stepped in.
In Pokémon Mystery Dungeon: Gates To Infinity , the reason that Munna and her gang are working to ensure that the world's foreseen destruction comes to pass is that all of them had suffered terribly at the hands of others (exactly how isn't elaborated on) and believe that a new world free of injustice and pain can be created after the end of the current one.
Schala, from the Chronogames. In Trigger, she was abused and neglected by her mother, Queen Zeal, who went mad after discovering the power of Lavos. Schala was later caught in the Ocean Palace as it collapsed and never seen again... until Cross, when it was revealed that, since she had such a miserable life, she wished for none of it to ever happen. This let Lavos take over her soul, and turn into the Time Devourer, which threatened to destroy everything that ever existed. She finally gets a happy ending at the end of Cross, when Serge uses the Chrono Cross to free her from the Devourer and Ret Gone Lavos out of existence once and for all.
The freaking MAN-AT-LEGS from Pikmin, believe it or not! Yep, that creepy mechanical spider that blasts your Pikmin to oblivion with a gun! How? According to its journal entry, the Man-at-Legs has no need for the gun since it has no natural enemies, leading to the rumor that the gun controls the creature! Let's look at it from the Man-at-Legs' point of view... imagine if you had a gun attached to you that killed everything around you that moved, and you were targeted by a giant swarm of insects for reasons beyond your control.
Pyrrha, the daughter of Fallen Hero Sophitia in Soul Calibur V starts out as an innocent, drifting Woobie with no home or family of her own; being kidnapped by Tira at a young age followed by the demise of her mother resulted in her spending the next seventeen years of her life searching aimlessly for somewhere to belong, regularly attacked and forced to kill or be killed due to Tira's manipulations to make her a suitable heir to Soul Edge. When she finally reunites with her long-lost brother Patroklos, however, things finally seem about to get better for her. Then Patroklos finds out Pyrrha is Malfested when she taps into her Superpowered Evil Side to save him from the Big Bad, Nightmare. The still gentle and mostly-innocent but now utterly terrifying Pyrrha turns to Patroklos in concern to make sure he's OK only for Patroklos to point his sword at her in the midst of a Heroic BSOD, finally running away from her in horror. Pyrrha, heartbroken, decides she can never trust anyone but Tira and willingly takes up Soul Edge to escape being alone. The resulting entity Pyrrha Omega, is extremely violent and deadly compared to the frightened and naive Pyrrha, but she screams as much in pain as in rage while she fights, and doesn't seem so much to be possessed by Soul Edge as she is madly lashing out at everything, thinking the entire world is her enemy and desperately needing to do something, anything to escape her pain.
Siegfried Schtauffen in the original Soul Edge and Soul Calibur, and partially in Soul Calibur II prior to his quest for redemption. A teenage bandit who accidentally beheaded his own father during a raid, Siegfried completely lost his mind and set off in search of his father's "true killer." On recovering the Soul Edge from the defeated Cervantes De Leon, the Evil Sword completely consumed the fragile mind of the boy, transforming him into Nightmare. Whilst most characters fall under the thrawl of the Soul Edge on possession of it, it is implied the sword influences them to do evil, corrupting them with its power. In Siegfried's case, his immature and unstable mind allowed the sword to transform him into an extension of the sword itself. Eventually as the sword is weakened by its defeat at the end of Soul Calibur, Siegfried's will begins to reassert itself until he is finally able to break free and begin his quest for redemption.
Iji has Iosa the Invincible. A maniacal Blood Knight even by her species' high standards, her lust for carnage began when the Tasen Alpha Struck her home planet, killing every living thing on it except for her. Afterwards, she desired nothing more than the extermination every last Tasen in existence.
Iji herself can become one of these, depending on the players actions. Indeed, Iosa is portrayed as her Evil Counterpart because their histories (and possible subsequent actions) are almost identical.
Mithos Yggdrasill of Tales of Symphonia definitely qualifies. All he wanted was to make a better world and try to get rid of the Fantastic Racism that plagued it while he was at it. After his sister dies, he spends the next four thousand years trying to revive her as he uses humans as lab rats for his plan to commit genocide by turning everyone into the same inorganic beings.
Arietta the Wild from Tales of the Abyss. First, her family got killed during the Hod War, and she's left to be raised by the Ligers. Then she got handpicked as a Fon Master Guardian for Ion, but quickly lost her position to Anise, not knowing that the Ion she loved is dead, and the current one is only a replica. Then, her mother got killed. And later, just when it looked like she may pull a Heel-Face Turn, Anise lets Ion get killed, which revokes everything Arietta thought about changing sides. Then, she challenged Anise for a final duel and... she's not even told about the truth about Ion, leaving her to die miserably and not realizing the reality.
The Big Bad of Radiant Historia turns out to be the previous Sacrifice, who was less than amused by the fact that his asshole of a brother got to be king (and do a terrible job of it) while he was expected to die to hold off the end of the world. His exploration of the various possible futures just led him to the conclusion that the world sucked so much he might as well destroy it, so he up and left. The fact that King Victor's reaction to this was to kill his brother's favorite nephew to get himself a new Sacrifice probably didn't help.
In the Disciples series, God of Evil is deconstructed since the "evil" gods only became evil because their "good" compatriots were jerks who treated them like garbage. This does not apply to their followers. Bethrezen's Legions of the Damned are by far the nastiest faction who gleefully corrupt and slaughter in Bethrezen's name and The Undead Hordes are utterly ruthless while following Mortis' will.
The Origami Killer, aka Scott Shelby in Heavy Rain is the way he is all because his own father was a complete bastard who regularly abused him and his brother both physically and verbally and ultimately convinced him he was worthless, and then to top it all off was forced to watch his own brother drown and said dad refused to help him. And his mom, the nice parent, apparently contracted Alzheimer's and all but lost her mind so she couldn't take care of him anymore. And now he drowns children and puts their fathers through sadistic tests all in the deluded hope that they can save them and that the kids can have fathers that are worthwhile and will always be there for them, something he never had.
General Nathan Sheridan, the Big Bad of Fracture. When you read his back story you find out he sacrificed his career in the Atlantic Alliance military, his reputation, his friends and family all for the sake of his daughters, who were diagnosed with a rare genetic disease which couldn't be cured by conventional means. Desperate, he moved to the Republic of Pacifica, where there is no ban on genetic research. There the best geneticists worked tirelessly in a race against time to save them, but their efforts were futile and his daughters died within weeks of each other. And just to twist the knife in a little deeper six weeks after their deaths a cure was found. His wife divorced him soon afterwards and the rest of his life collapsed around him. Determined to make sure what happened to his daughters never happened to anyone else, he starts building an army the likes of which the world has never seen before to remove the ban on genetic research by force.
Yuriko Omega from Command & Conquer: Red Alert 3. Found to possess psychic powers from an early age, she was scorned by her schoolmates, and eventually taken away from her family by the Japanese government to be placed into a weapons development program, which put her through Training from Hell to develop her inherent psionic powers, at the cost of her sanity. When World War 3 broke out, she was forced to fight by the Japanese military, who viewed her as little more than a weapon. After the defeat of Japan, she was captured by the Allied Nations, only to break out, partly to get revenge on the head scientist of the project, and partly to meet her "sister" Izumi. This sets off her campaign in the Uprising expansion. While she eventually manages to kill the scientist and get her revenge, it turns out Izumi was always planning to kill her, and Yuriko is forced to kill Izumi in self-defence. In the ending cutscene to her campaign, she is shown alone and depressed on a hillside, overlooking a city, pondering what to do.
The Hecatomb in Phantasmagoria 2 is revealed to be the original Curtis Craig, thrown into the alien world when he was young by PAW and forced to grow up in a hostile alien dimension. He wants to destroy alien Curtis and kill his friends out of revenge.