Finn of AJCO suffered one before he even appeared in the AJCO storyline, having once been a perfectly normal human that was cursed, killed, turned into a man-eating demon, and then accidentally summoned and bound-by-soul to a clueless witch. He does his best, but the poor guy has been through a lot.
Three-eyed, foul-mouthed cactus alien Kube may not be a conventional "cutie", but they become pretty upset when Vinnie gets their throat ripped out by the above-mentioned demon, snapping at people that tried to help them and fretting constantly. Seems to be getting better having made amends with Egg, though Vinnie's situation hasn't improved much.
Ask King Sombra has this happen to Coffee Talk via her situation and Sombra's Enemy Within giving her a Breaking Lecture and Caribbean Splash when his best friend Whistler is killed in the fight with the Crystal Wolves.
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 Frollo Gets Flashed by a Gothic Lolita, she witnesses numerous deaths of both allies and villains alike during the final battle. Later, she struggles against her fight against Wilford Brimley, while recalling those moments. Wilford then rubs salt in the wound by asking where her mother is, triggering flashbacks. After that flashback, she summons an alternate dimension to kick Wilford's ass in... but the way she did it looks like she was going to kill herself.
It gets worse for her later on. The next episode, Wilford had created an entire dimension after absorbing her sorrows, and the only way for Frollo and his allies to find him is to break her even more, sometimes by accident or forcefully (the latter giving a What the Hell, Hero? moment to Frollo from even the villains).
Ib suffers in the same episode when she witnesses Batiatus killing Garry. This enrages her and prompts her to deliver a brutal beatdown on Batiatus, only for Batiatus to kill her in the end. Stocking and Madotsuki slaughter him in response, the former appalled at his actions.
In The Gamer's 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.
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 teammate Helix, a sadistic psychic, had already done their worst to Pixel, leaving her a sobbing mess.
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.
The first is Season 3 finale when Roxana kills his pets in front of them.
It happens again to a lesser extent in the Season 4 finale during Arthéon's "Reason You Suck" Speech about the guild.
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.
The Nostalgia Critic. Apparently he was a sweet enough kid to get into the 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?
Before Season 9 of Red vs. Blue, few would guess 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), randomly found a dead friend—who he thought died many years before in a completely different place, and is constantly betrayed by nearly everyone he knows. If there's one thing that goes with Wash, it's tragedy.
Poor Maion from The Salvation War 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. 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.
This viral video by Save the Children UK shows the effect of war on a little girl through a year's worth of single-second clips.
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.
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.
Many, many innocent people in Worm undergo life-changing trauma, but the nearest thing to this trope during the time of the story is probably Parian.
Bonesaw also underwent this in her past. The SlaughterhouseNine broke into her house and attacked her family, causing her to gain her surgery powers. She was then forced to heal the injuries the Nine inflicted on them, only for the Nine to hurt them again, and so on, for hours, which continued until she had to let them die from sheer exhaustion. On top of that, they forced her to say she didn't love them. Afterwards, she joined them to survive, leading to her Becoming the Mask.
Toki's past, if they are to be believed, started as this, particularly when she tried to run away from them and, as Brownie put it, she was once sweet and only wanted to be loved. Let's see. Here they are:
She was abused and then neglected, causing her to almost die of leukemia during the events of "Flashbacks I", something that caused something in her to drive her insane in the next incident (see below).
She returned to the place where she was once abused so she could get ahold of the inheritance money, but went insane during the events of "Insanity and Resentment" and has had issues ever since.
Frailine (the one Toki abused) is also an example as per these statements:
She was imprisoned for being criminally insane after the events of "Little Girl's Revenge" when she almost killed Toki and damaged the border between this world and the next. Kind of a Disproportionate Retribution, compared to what Toki did to her (abused her and then, sometime afterward, sent her to Alaska attached to a bus by a magnet with no means of getting home).
Sometime after being imprisoned, she almost kills herself twice and is never seen again, as stated in "Diary of a Borderline Schizophrenic", driving Toki into more self-hate because of it.
 Laughing jack from Creepypasta Origin stories can also be considered this, all he wanted to do in the beginning was help a sad little boy smile, but due to a series of events, the boy was sent to a boarding school for a number of years, leaving Jack locked in his box for years, until he was corrupted/driven insane (Depending on your interpretation) from being left in there for so long.