In the doujinshi Besame Mucho, Spain gets one of these (quoting the trope name included) when standing in front of a destroyed house in a village his army just slaugtered. He is passing by when he sees a dead child's arm peeking out of a pile of wreck, and it makes him remember about Romano. Something was in my eye when he says that they just killed people, who were the same as him or little Romano. His companion's answer? "You are wrong. You are not human.". Tear Jerker, indeed.
Also in an Axis Powers Hetalia fic, this is lampshaded by an authoress in the notes of the first chapter of her intense FACE bondage-dominance foursome. Repeat, the /first/ chapter.
In yet another Axis Powers Hetalia fanfiction Let me be Your Savior Romano/Lovino has one after stumbling across his brother (Italy/Feliciano) after Romano/Lovino had violently raped him crying, and it finally hits him that what he's been doing is wrong. This scene is a nice breakthrough, seeing as Romano/Lovino had seemed to cross Moral Event Horizon before.
A Protector's Pride has Yamamoto himself go through a subtle version of this while waxing philosophical as he realizes that his playing Obstructive Bureaucrat and treating Ichigo as one of his soldiers has led to Ichigo only tolerating the Gotei 13 while they have been giving him reasons to outright hate Soul Society and watch it burn.
In the Jackie Chan Adventures and Teen Titans crossover fanfiction A Shadow Of The Titans, Jade has a reaction like this when she severely wounds Kitten during their battle in the Tournament of Villainesses (to be fair, not only was Kitten really rude to Jade when she was SEVERELY outclassed AND offered a chance to surrender, Jade, due to her previous encounters with the Titans, etc., expected her to be Made of Iron). Of course, the other villains think her reaction means something else.
Twilight has a mild one after killing a Dalek, if only because she had previously thought it was an robot instead of a living creature.
Rainbow Dash has one when she realises that her Buccaneer Blaze tore Pikachuto shreds - she had been ready to kill in self defence, but she hadn't meant to be quite so brutal.
Pinkie Pie has one after beating Taokaka, as the two had gotten on well up until Discord showed up with a horribly inaccurate video of her battle with Lucy.
Fluttershy has this reaction in chapter 10 of Ace Combat: The Equestrian War. She saved Cloud Kicker's life, but believes her actions partially led to Razor's death. She utters the actual quote (without the "My God" part) and until her friends comfort her later on, along with Princess Celestia stating that Razor died in the explosion caused by The Burning Talon's overheating, she is reduced to cryingand blaming herself.
In the Tamers Forever Series, when Chaos first emerges, but before Takato realizes he's a separate entity. From his perspective, he destroyed a significant part of the Internet just by having his Berserk Button pressed. You'd be a bit scared of yourself too.
This is also Takato's reaction when he uses the Omnivice to remove the pain from Azulongmon's Wind Electricity technique, only to find out that he'd transferred the pain to Rika, Henry, Renamon and Terriermon. His horror, frustration and grief combine to send him over the edge, unleashing the power of ChaosGallantmon.
In The New Retcons, after Elly snaps out of her insanity and is confronted by the doctors with what happened to her and what she did to others, she's horrified at her actions, particularly when she reads the letters she wrote while insane, and realizes how much Connie enabled her.
In The Day He Was Pushed Too Far Dumbledore, in order to avoid legal repercussions, let a copy of Harry "die" and gave Harry a new identity after Harry shot Umbridge and then himself fifth year. Harry said "What have I done?" shortly after coming across Neville, who was dealing poorly with the "loss" of the only person who cared about him, trying to kill himself.
In Lily's Changes Voldemort said this almost word for word after eleven-year-old Harry, having future knowledge, convinced him to absorb the soul fragments in two of his Horcruxes, regaining his sanity.
Sora in Broken Heroes has this reaction twice: When Ventus dies and he realizes how much of a Jerk Ass he has been to him, which causes his character 180, and when he agrees to become Xemnas's slave (Implied to be Sex Slave) to keep Riku, Kairi, and his other friends safe, which spurs him into adopting the identity of Roxas.
In the My Little Pony: Friendship is Magic fanfiction, Maternal Instinct, in a fit of unbridled rage, Queen Chrysalis exploded in a vicious tirade against her heavily mentally and physically disabled daughter and heir, Princess Pupa and proceeded to beat her. Savagely. But when Chrysalis came to her senses and saw her daughter on the floor, barely conscious and with blooding oozing out from wounds on her crown and temple, the aghast and repentant Queen immediately threw herself at her daughter's mangled form, kissing her face and trying to stop the bleeding, mentally crying, ‘Kami above, forgive me! Please, forgive me!’
After The Gala shows this as Fluttershy's reaction to her explosive display of anger at the Gala. She's outright horrified by what she did and left a sobbing wreck. Her friends console her, though, and manage to get her out of her Heroic BSOD both by their forgiveness and Celestia revealing everyone was okay and it wasn't entirely her fault (the animals were rather wild to begin with and Celestia apologises for not warning her beforehand).
"I do not need to be told the heinous nature of my actions!"
Subverted, however, because the author doesn't portray her as being entirely in the wrong for taking said actions. For those not familiar with the story, Celestia has been forced to separate a young filly from her adoptive mother -hopefully temporarily- in order to perform a complex diagnostic spell on her, acting against her better judgement but on the advice of an expert in the dangerous form of magic that the filly in question had been touched by. This would have been unpleasant but justifiable, save for the fact that Celestia did not find out that the person giving her that expert advice was being economical with the truth to further his own agenda until the shit hit the fan.
The spy Lord Eminence in Under The Northern Lights when he realizes that his plot has been found out because he has been very careless because his special talent for being inobtrusive and even turning invisible doesn't work on reindeer, meaning there are lots of witnesses to his acts. He acted alone, convinced his success would force his superiors to accept a job well done, but Princess Luna is not only against the plan per se, she is also furious that he was so careless as to be found out. He doesn't say the actual phrase, but falls to the floor and moans "Mare in the Moon!" which prompts Luna to chastize him further for taking her name in vain when she is standing right there.
Twilight has a few of these as she copes with the fallout from Hard Reset. Especially given how she started treating ponies (herself included) during the Groundhog Day Loop.
According to A Brief History of Equestria, this was Smart Cookie's reaction when she realized that the plans she helped Puddinghead draw up for colonizing a new homeland would mean effectively leaving the other tribes to slow death by starvation and hypothermia.
A Future of Friendship, A History of Hate: Rarity has this reaction in Episode 4 when she realizes that she's been blowing off her friends without a thought in order to spend time with Regal Rule, most prominent being that she completely forgot about Fluttershy's birthday.
Asuka, who has been physically and sexually brutalizing Shinji for months, has minor moments of this whenever her temper finally abates, then has a massive moment of this coupled with Villainous Breakdown when she finally beats Shinji badly enough to nearly kill him. The fic starts after this moment; when it's not filling in backstory, it's detailing her attempts to atone for her actions.
Shinji, on the other hand, blames himself for everything Asuka has done to him, seeing his actions during the canon story (especially the hospital scene in Eo E) as so unforgivable that he can justify her abuse as karmic retribution.