In the Bleach fanfic A Protector's Pride, Ichigo says this about Aizen. Ichigo claims that Aizen might have wanted to change Soul Society for the better, but he unfortunately picked up a God Complex. Along with his inability to talk to others as equals, this led to the monster he became.
In the Jackie Chan Adventures and W.I.T.C.H. crossover fanfic Kage (part of Project Dark Jade), has one of these it the backstory, long ago, a disciple of Kandrakar, Althair the Mad Sage, led a crusade across the Known Worlds, killing thousands (especially children) in order to prevent Oracle N'ghala's vision of the "Incoming Darkness" from coming to pass.
Nerissa believes she is one, but thinks that the best way to a peaceful universe is through obedience to her and her alone.
Twilight Sparkle, as Kira, has set upon a quest to eliminate all crime in Equestria, by killing all criminals. And she's willing to kill innocents as well, in order to protect herself.
Fluttershy, the Second Kira, likewise believes in Kira's message, and is willing to kill anypony who speaks against her. Twilight objects to this.
L is willing to go to any lengths to stop Kira, no matter how morally objectionable.
Ace, leader of the Kira Neutrality Movement, believes that the only way to stop Kira is to remove her motivation (crime), and is thus pushing for harsher laws. That this would border on turning Equestria into a Police State and in no way prevents Kira from being a Karma Houdini are acceptable consequences to him.
An interesting case in Dungeon Keeper Ami. Both Empress Mercury and The Light qualify in different ways. Mercury wishes to return home, despite bonding to a Dungeon Heart, wich means if she succeads she will draw the Dark Gods after her. The Light knows that Mercury is an unwilling Keeper and has no intention of harming her native world, or the world she now lives in. However, they see no alternative but to oppose her as there seems to be no way to help her without also helping the Dark Gods.
Red Eye is this, and naturally fits Face-Heel Turn too. He started his own personal quest to rebuild Equestria, probably long before Littlepip even found her Cutie Mark, but became so driven to reach this huge goal that he let go of his morals. Soon, he had built up an empire upon the backs of slaves, hired merciless mercenaries to protect his assets, and forged a deal with The Goddess (which he planned to break once he worked out how to become a God himself, and kill The Goddess), who was a genocidal maniac herself. All of the things he did, no matter how horrible, were done to achieve the noble goal of rebuilding Equestria and bettering the lives of its inhabitants.
Littlepip can classify as one too, or at least she have shades of it. Scootaloo, on the other hand, is a more straight example.
Averted in Kyon Big Damn Hero. Koizumi absolutely refuses to use his new powers on unwilling subjects, seeking consent before experimenting on them, which makes it difficult for him to practice his powers.
The Order Triune in The Dresden Files and My Little Pony: Friendship Is Magic crossover fanfic The Dresden Fillies False Masks see themselves as this. They seek to wipe out any trace of a reincarnated Evil Overlord. This is all well and good, until they've mistaken Harry Dresden, someone they've never even met, as this reincarnated villain. In the process of trying to kill them, they've kidnapped Twilight and Trixie, spied on many ponies, released a hydra that could have killed bystanders, and summoned He Who Walks BehindTWICE which could have easily killed the surrounding ponies. In their final act, before they were themselves sacrificed to summon a demon would have led them to battle the very Princess and the Bearers of the Elements of Harmony, whom they claimed to be protecting.
Alphamon in Transcendence: Digital Curse seems to truly want to protect the Digital World. However, he believes that every problem that has endangered it originated within the real world. As a result, he wants to completely sever any links the two worlds have to each other. Gennai notes that if such a thing were to actually happen it could even destroy the digital world, but it's unknown if Alphamon actually knows this fact as well. So far, none of his actions seem to have contributed much to this goal. His attempts to eliminate Agumon even seem to be just to spite humanity by stopping them from having a relationship with digimon.
In Harry Potter fanfic Reparations, Draco works in the St Mungo's Department of Magical and Chemical Dependence (Chem Dep for short), which is basically rehab for magic folk. Most of the patients are addicted to a wizard drug called Chromia, but there's also a new drug called Chromia X, which is stronger, more addictive and thus more dangerous than regular Chromia. It turns out that the department is constantly running the risk of being shut down because the the board doesn't understand its importance, so the head of the department, Draco's boss, creates and distributes Chromia X because he figures that if the number of patients increases, the board can't justify shutting down the department.
Damballa in the Horseshoes and Hand Grenades story A Month of Sundays, transformed Shotaro, Haruto and Eiji into Creepy Dolls because he has been trying to find a way to revive his own grandson who got lost in the struggle of the War of the Stars. Unfortunately, he's forcing the transformation on them, shattering their minds in the process and he truly cannot grasp the idea of a human mind. Thankfully, Jun helps him see the light and reunites him with his grandson.
In The Shrouded Path, Vanitas, the Big Bad behind Silent Hill, is responsible for the torture (and often death) of the people who enter the town. However, he honestly thinks he is doing the universe a favour by ridding it of those people's evil.
Inner Demons: Scootaloo, having performed a Face-Heel Turn and joined up with Twilight (who herself had been consumed by her Queen of Darkness persona and become the Big Bad), is this mixed with The Starscream. She admits that, unlike Trixie and Sweetie Belle (who both also had Face Heel Turns around the same time she did), she's known all along what a monster Queen!Twilight is, and has only served her in order to get close enough to kill her and end her rule over Equestria. However (and this is where the Starscream angle comes in), she is clearly enjoying the pain and suffering she inflicts on the heroes, and more to the point, she has no intention of returning the throne to the Princesses — once Queen!Twilight is dead by her hoof, she intends to set herself up as the great Hero who saved the day, and then take the throne for herself.
Nightmare Night and Nyx: The conspiracy trying to eliminate Nyx are all convinced she's still evil and has brainwashed the Princesses, and thus has to be destroyed. They all have a collective My God, What Have I Done? moment when they realize how wrong they were.
Pages of Harmony also portrays Twilight as this. All she seems to want is for Harmony to reign in Equestria.... which she plans to achieve through performing torture and Mind Rape on her friends to extract their Elements, as well as anypony who gets in the way of her plans.
Rolling in Beaches: King Kenbroath and his clan honestly believe that ponies are descended from cursed dragons (it's actually the other way around) and that they must use the Rainbow Pearl to revert it. He doesn't care to ask anypony's permission first because he figures granting them all substantially longer lives and great power would be a blessing.
Ho-oh from Poké Wars wants Pokémon-kind to live in a utopia. He plans to achieve this utopia by engaging in a genocide against humanity. He later goes Jumping Off the Slippery Slope by doing things like ordering wanton killing of Pokémon, contrary to his ideals, utterly ruining the environment and not giving a damn about it, and generally being a filthy hypocrite.