This trope is not only performed brutally and horrifically in Broken Saints, it's part of the Big Bad's plan: Shandala is raised in paradise by a loving, nurturing family, then taken away from said paradise and had bad thing after bad thing happen to her, all so that her empathic abilities would become fully realized, and used to the Big Bad's advantage.
In LessThanThree Comics' Brat Pack, resident Cutie, Pixel, tries to take on the psychopathic villain with a grudge, Gauss, all alone, while her teammates go home to change into their uniforms. Naturally, when they find out who she's fighting (the team's precog only knew something big was about to happen), they rush back, but are too late, Gauss and his own teammate Helix, a sadistic psychic, had already done their worst to Pixel, leaving her a sobbing mess.
The Nostalgia Critic. Apparently he was a sweet enough kid to get into boy-scouts but his parents were abusive enough to be drawn as monsters, his eighth grade classmates pitied him for being a brat and he hid in the cupboard whenever he got scared. He once broke up with someone three times and they went psycho-stalker on him and he got date-raped (and it would have to be similar to how Spoony acted on Spooning With Spoony 2 so... ouch) on his prom night. With all that, wouldn't you be cynical and not doing so well on sanity?
Hammered home in the Commercials Special. When he's on the verge of giving up completely, he takes out a picture of him as an eight year old and angsts about how he had such promise. But then, heunbreakshimself. At least for the moment, he's "pathetic but proud".
The Nostalgia Chick was a sweet kid too, with friends, a love for honorable men and wanted a huge wedding when she was six. But due to a lot of disappointments, she's now a lonely alcoholic who needs to abuse people to make herself feel better.
Poor Maion from The Salvation War series gets hit with this trope bad, she's a young angel who when we first see her, has been addicted to heroin and forced to beg for more, then she gets forced into exotic dancing and later prostitution, then she ends up in a concentration camp where she gets tortured to the brink of death and she and her mate have to flee to Earth to seek medical treatment from humans, oh and then it turns out her shattered wings are never going to heal properly and they have to be cut off, fortunately for her, angels have a Healing Factor in this 'verse and they grow back
Viridian Dreams Quests - Vampire Quest is basically one long string of heartbreak with the player character we, in our foolishness, made a lovely, normal girl trying to bring a little happiness to the Crapsack World setting. Surrounded by pre-broken cuties and heading that way herself very quickly.
The Cerebus Syndrome laden fourth step of There she is!! just devotes itself to breaking poor Doki. As if getting harassed for dating Nabi wasn't bad enough, she gets beaten to a pulp by an angry mob, who (whether it was an accident or not is never made clear) kill her pet hedgehog, and just to top it off, Nabi dumps her for her own safety.
Hibiki Outori from a Naruto RP. Similarly to Sasuke Uchiha from the canon, Hibiki was a sweet, naive child, loved by his parents and just wanted to do his best. After witnessing his parents murder, he not only killed the murderers, but now flashes back to it from time to time, and, aside from his teammates and his adoptive mother, he can be quite cold and heartless, which he certainly was not as a child.
I doubt anyone would have guessed before Season 9 of Red vs. Blue that Agent Washington was anything but a stone-cold, die-hard badass. The more we see of him pre- Epsilon unit, though, the more it seems like he was a Wide-Eyed Idealist who was even kind of Adorkable before he was broken beyond repair between seasons.
Then one realizes the breaking kept going even after the Epsilon breakdown and he hid every bit of it. For the rest of his freelancer career he was hated by most every other agent in the project, was forced to harvest equipment from their corpses after they died, and kept working for a project he hated more than anything. Then, he had to brutally fight his former best friend on multiple occasions (eventually leading to that friend's death), found his former lover dead in the sand, and is constantly betrayed by nearly everyone he knows. If there's one thing that goes with Wash, it's tragedy.
In The Gamers Alliance, Marya Aurelac is a kind character but after her husband Kagetsu I's sealing she is forced to live her life as a wandering prostitute. Things get better for her for a while when she is reunited with Kagetsu but then the warlock Arawn kidnaps and rapes her both for pleasure and to keep Kagetsu from rebelling against him. Marya is saved but only gets to spend a little while with Kagetsu who is later killed in battle. Marya is an immortal but her beloved Kagetsu is now dead so she realizes that she has to live her life without him for all eternity. This finally makes her snap, and she turns into a cold, ruthless individual who later kidnaps and corrupts Prince Geraden Aurelac of Maar Sul, her descendant, going even so far as to boink him in order to control him. Once she's made sure Geraden can take over Maar Sul without needing her to guide him any longer, Marya suffers from guilt of all the nasty things she's done and finds a way to properly commit suicide with poison which bypasses her curse of immortality in order to finally be reunited with Kagetsu in the afterlife.
