Sandra Eastlake from Zebra Girl. After trying for two years to come to terms with accidentally being transformed (through no fault of her own) into an obviously non-human demon who cannot eat or even taste food properly, type properly (her fingers are razor-sharp, foot-long claws), sound normal (her voice sounds like the modulated noise of a cat being strangled), interact within normal society, or even hold a normal job, surviving multiple attempts on her and her friends' lives, being rendered incapable of intimate contact due to her bodily fluids being acid, and becoming her town's very own urban monster legend, she is finally pushed over the edge by a wizard who goes out of his way to provoke her into becoming completely evil so that he can successfully poison her with an evil-killing toxin. When even that fails, he simply drags her to Hell while the whole town stands by and does nothing, because he happened to look more human than she does. She finally snaps when, after they both arrive in hell, he transforms into a demon and threatens to spend the rest of eternity torturing her. By that point, she has nothing left to lose. And as icing on the cake, consider that the only reason said wizard even knew about her in the first place is because she contacted him to ask for help.
Sluggy Freelance's Oasis. All she wants is for her one true love to return her love, and until he does, she'll slaughter everything in her path. She knows that she was essentially brainwashed by Steve Hereti into loving Torg, but can't help it anyway. This, coupled with her not knowing exactly what she is, is slowly driving her insane(er).
Trace Legacy from TwoKinds. A few weeks after he settles down with his new wife, a Keidran thief shoots her with a poison arrow. In his grief, he throws himself into Black Magic research to try and resurrect her. The spell backfires, nearly killing him and destroying his sanity. He then sacks a Keidran village and takes over the Order, polymorphing his old boss into a Keidran so he can kill her without a fuss. The Goddess of Neutrality has to wipe his brain to keep him from committing genocide.
Tavor from Looking for Group. After losing his family and kingdom to invaders, he decides to take his pain out on the rest of the world by trying to erase the city representing its last hope from existence.
Redcloak once you've read his backstory in Start of Darkness. Entire family and village, except his brother, massacred by the supposedly Lawful Good Sapphire Guard. Teamed up with Xykon, who took over Redcloak's own team and demoted him to number 2 after Redcloak himself turned him into a Lich. Forced to kill his brother by Xykon as a twisted loyalty test. Recently lost an eye to one of the Sapphire Guard and was forbidden by Xykon (again) from healing it. He remains in service to a potentially world-destroying evil plan which is the only part of his old life that remains.
Darth V also qualifies, having snapped after being unable to trance for months due to severe guilt, then having his/her family threatened with a horrible death. The fallout of his/her casting of"Familicide" on the dragon making these threats, killing everyone even remotely related to her, has indeed pushed this a lot closer to a literal "Destroyer of Worlds" example — one of the Gates sealing awaythe Snarl was defended by a man with dragon blood, and his family.
Eridan Ampora from Homestuck. The first thing he does when he's introduced? Shoot a laser through a whale. Second? Have all of his romantic advances rejected when desperately pining for companionship. The last thing he does while alive? Kill his queen, kill his friend, knock out his rival, and doom his species to extinction.
It's not hard to feel awful for The Handmaid's pre-scratch incarnation, Damara Megido, who started as a sweet, meek young girl and snapped after being bullied incessantly over a bad breakup, which itself was sparked by her boyfriend cheating on her. Not that it makes her actions excusable, but it's still heartbreaking.
How about Gamzee? He's very friendly and silly throughout Part 1 of Act 5, and only goes Ax-Crazy upon realizing that the Juggalo religion he followed was really a musical project by a couple of Earthlings, and thus stops eating the sopor slime which kept him sane. It may sound ridiculous on the surface, but when you compare it to human examples of the Faith–Heel Turn, you can't help but feel sorry for the poor guy.
It's also possible to feel pretty bad for the Condesce, who spent most of her apocalyptic reign over Earth attempting to recreate the Troll society she'd essentially been banished from, desperately trying to reconnect with her own kind, only to have her every attempt fail miserably until she was left to rule over a devastated planet. You'd feel very sorry for her if she hadn't taken down humanity in the process.
The golem girls in Wapsi Square played this role once. They were tortured until their wills were completely broken, then they were turned into a weapon. They ended up rampaging and destroying most of the world. Since then, civilizations have been rebuilt, and they have had their free will returned.
In 151 Hidden Depths, Diglett becomes this after evolving into Dugtrio. Being small led him to be rejected from joining the Pokemon Police Force so he proves he's a force to be reckoned with by destroying cities.
In parent series Roommates Jareth actually destroyed a (thankfully just dream)world, when he got pushed too far in the Such Stuff... arc. (His mother manipulated his Superpowered Evil Side into killing his best friend... and no it wasn't a redeeming point that she was aiming at his Love Interest either.)
Shadow the Hedgehog from Sonic the Comic – Online! was created by the Drakons during the war, but his genetic make-up was never completed. After the war, the incomplete Shadow was left in Emerald Hill Zone, along with the Chaos Emeralds for hundreds of years. There he would remain, until he was eventually discovered by Doctor Kintobor. While Kintobor could never work out how to give Shadow a physical form and free him from the Omni Viewer-like container he was trapped in, he taught Shadow all he knows, including geography, science, history and politics leading Shadow to consider Kintobor to be his only friend. Then Sonic turned Kintobor into Robotnik, dooming Shadow's only friend, which lead to Shadow working to carry out his friend's last wish to destroy Mobius.
Applejack is grievously wounded and placed in a cyborg life support suit by Rarity, who is unaware that her friend has been touched by the Nightmare Force. Part of the suit is a mystical artifact that was supposed to reinforce Applejack's mental fortitude: instead, it allows the Nightmare Force to take her over by convincing her that she is never wrong... even when the voices in her head tell her to cover an entire planet in Smooze.
Rarity, having already been a Nightmare, can't be corrupted again - but when faced with bigoted government policies directed at innocent pegasi, she encourages them to secede from the Empire, starting a galaxy-spanning war.
Fluttershy is lured to Nightmare Force with promises of the power to end the Pone Wars (which Rarity had pressured her into fighting in). As a Nightmare, she wipes out the Alicorn Princesses, bringing the war to an end when she breaks Princess Luna's horn, thus removing the Confederacy's strongest asset.
Stand Still, Stay Silent: "Sleipnope", the powerful entity that has its mind set on killing every single member of the crew, is made of several beings undergoing a Fate Worse Than Death. While its "main" voice is uttering And Your Little Dog, Too! threats to the crew's mages (the only ones that can hear them), other voices can be heard calling for help.