Chapter 95 of : Roy is taking brutal revenge on Envy when Ed, Scar, and Riza step in to stop him. Roy asks Riza what she plans to do after she shoots him. The answer? Kill herself. He backs down immediately.
Kanaka in Narue No Sekai. A victim of Time Dilation, she's missed a couple of decades. After she realizes that everyone she knew is now completely different, she freaks out and tries to run off. Narue, determined to befriend her (technically older) sister, runs after her. Kanaka pulls a laser gun to try to frighten Narue off, and after a brief tussle, Kanaka completely unintentionally shoots her. Fortunately it was a non-lethal gun to begin with, but it's still enough to get Kanaka to thoroughly regret her actions. Well, that and some manly tears from Kazu.
In GUN×SWORD, Michael, who has become a loyal minion of the Claw meets up with his sister, Wendy, one of the heroes. Wendy ends up shooting him in the arm to try and stop him, and he starts strangling her. Then, her pet turtle bites his finger and he snaps out of it, realizes what he's doing, and saves Wendy when the lair starts to collapse. While the series implies a Heel Face Door Slam for Michael and his lover Fasalina, who likewise snapped out of her loyalty to the Claw both appear to be crushed under rocks and averting Just A Fleshwound, Wendy's shot is implied to be fatal, supplemental materials go for Redemption Earns Life.
In the Sands of Destruction manga, Morte finished her job in a city where the beastmen were eating humans. After berating Kyrie of doing something reckless, she blows up the entire city that they just saved for the fun of it. This action causes Kyrie to snap at her and he leaves the group which causes the group to split up (with Toppi and Naja going with him and Agan staying with Morte.) When he comes back, he throws the gun that she gave him to defend himself on the ground telling her that he doesn't need it anymore since he's not coming back. Cue, she snaps and attacks him proving Kyrie's point why he can't stay with her. Thankfully Toppi and Naja stopped her attack.
In Elfen Lied, Lucy brutally kills Kouta's sister Kanae in the middle of what's basically a mix of a psychotic breakdown and a hissy-fit of jealousy. While she's still murderous after that due to other unrelated events, quite a bit of guilt over said incident remains in her heart.
Subverted in Durarara!!. Everyone assumes that Izaya did this to Shinra in middle school for no reason whatsoever. Turns out, he told Shinra to tell everyone that he stabbed him so he can use this opportunity to ruin Nakura who even dare hurt his only friend. Naturally, it's implied that Izaya also did this so no one will find that how close he is with Shinra.
When Sakura sees Naruto's fox form for the first time, she tries to talk him out of it. Problem was that Naruto wasn't Naruto anymore; he was controlled by the Kyuubi, resulting in a horrifying 4-tailed state. To simply put it, he tail-slapped Sakura on the shoulder (badly) and tried to kill her while she was down.
In Puella Magi Madoka Magica, as Sayaka begins her downward spiral, she ends up upsetting Madoka when she angrily lashes out at her for not using her incredible potential as a Magical Girl to aid her in the fight against witches. Unfortunately, this only makes Sayaka feel even worse, and she even rejects Homura's offer of a Grief Seed, given with the sole intent of preventing Madoka from being hurt by Sayaka turning into a witch.
In Girls und Panzer, this is an unfortunate result of Maho adopting a Jerkass Façade toward her younger sister Miho, doing everything that is expected of her as heiress so that Miho can live her life her own way. When Miho confronts her about shooting an enemy flag tank that was trying to save her own team, Maho coldly dismisses her, leaving her in tears. When Mihoagainst her typical nature, confronts her mother to ask her the same question, Maho realizes how much she has upset Miho and apologizes.
In Villains United, Cheshire was blackmailed into joining the Secret Six with threats that a bomb was implanted in the back of her daughter's head. By the end of the miniseries Cheshire conceived a replacement child via seducing Catman. Neither Lian or her father Roy ever learned about this, which allowed Cheshire to continue her emotional manipulation of Roy long after Lian's death.
Just like in the Animated Series, Spider-Man 3 has Peter Parker turn into a douchenozzle thanks to the alien symbiote turning him aggressive. He eventually goes on a date with Gwen Stacy (after getting dumped by Mary Jane), walks into the bar MJ is working at and proceeds to enter a song and dance meant to humiliate her. Finally the bouncer gets fed up and tries to throw him out. Peter easily deflects him, but also instinctively knocks MJ down when she tries to grab him. Peter suddenly realizes the symbiote is controlling him and tries to get rid of it.
Blade Trinity starts with the vampires arranging a Kick The Wrong Dog moment for Blade: The "vampire" that he was chasing simply had fake teeth.
At the end of Star Wars Episode III: Revenge of the Sith, Anakin Skywalker force chokes Padmé Amidala nearly to death when she tries to convince him what he's doing is evil. Unfortunately, the incident doesn't prompt him to stop, instead he just blames Obi-Wan Kenobi for turning her against him.
X-Men: First Class includes a dramatic confrontation where Magneto is deflecting bullets from a CIA agent trying to stop him from destroying the US and Soviet fleets. One of the bullets flies off and hits Charles in the spine. For a moment there's a look of pure horror and guilt on Eric's face, then he decides it was the CIA agent's fault (Xavier vocally disagrees and talks him down for the moment by playing on his guilt).
This is Albus Dumbledore's backstory in Harry Potter and the Deathly Hallows. He was dangerously close to joining with Grindelwald and becoming a Big Bad when his sister was accidently killed during an argument. At that point he came to his senses and instead became the Big Good that he is known as.
Also the story for Regulus Black. He was a loyal Voldemort supporter, proud to offer the use of his beloved House Elf...until he finds out that Voldemort wanted the elf to test the defenses around his horcrux, which involved the elf being horribly tortured and left to die. After he found out about that, he gave his life to try to bring Voldemort down.
And Severus Snape's. When he found out the information he provided would lead directly to Lily Potter's death, he switched sides. Pity she died anyway...
In Les Misérables, Jean Valjean steals from a little boy from force of habit after the bishop pardons him. While the child wasn't someone he knew, how monstrous the act was, especially since it came immediately after the bishop had pardoned him, was what prompted his Heel Realization and finalized his transformation into The Messiah.
Angel does this (sorta) with Gunn and Wesley fighting over Fred, but when Gunn goes to hit Wesley, he accidentally hits Fred instead. Whoops!
Whilst elsewhere in the Buffyverse, Willow's downward spiral of magic addiction is brought to a halt when she crashes her car with Dawn in the passenger seat while under the influencenote of magic. Her manipulative friend Amy, who has been encouraging her magic abuse, is hurt by Willow's decision to quit magic and eventually becomes a Big Bad in the Season 8 comics.
In the Year Inside, Hour Outside series finale of Stargate SG-1, Daniel, having gotten fed up with what he thinks are Vala's intentionally shallow attempts at flirting and seducing him, lashes out with a nearly minute-long rant about her shortcomings that reduces Vala, who actually is genuinely attracted to him, to tears. It's especially shocking coming from Daniel of all people, but a close listen as his rant goes on reveals that he shifts from disparaging her actions to voicing his own insecurities and hesitancy about them having a relationship. And when a disturbed Daniel realizes both this and how badly he's hurt her, he quickly lets Vala know that he does return her feelings. They then decide to see if they can sustain a relationship, and you can guess what happens next. And considering that except for Vala's having a miscarriage, they spend the next fifty years together with no problems before Sam reverses time, apparently they work out any remaining issues.
In the Saved by the Bell precursor show, Good Morning, Miss Bliss, one episode features Zack trying to join a club of popular jerks called the Rigmas. They tell him to start bullying his friends as part of his initiation. The biggest Kick the Morality Pet moment comes when he yells at Screech "Why would I want to be seen with a nothing like you?!" Thankfully, Zack later realized the error of his ways, and his friends forgave him. They together spend the last few minutes of the episode remarking on how stupid people often act in order to fit in with various groups. The episode ends with Zack symbolically dumping the bullies club membership jacket in the garbage while his friends give him the thumbs up.
In the climax of Power Rangers Time Force, the Big Bad Ransik has trashed the rangers and is getting ready to finish them of, when his daughter Nadira (having gained an appreciation for human life after helping deliever a baby) tries to talk him down. He ignores her, and during the fight she gets caught in one of his blasts. Horrified at having nearly killed his own daughter, Ransik voluntarily surrenders himself to the rangers.
In Breaking Bad, as Walt goes further and further towards/past the Moral Event Horizon, he starts to doing this more and more. By Season Five, only his children are safe.
Subverted in BIONICLE. When Vakama turns evil and Matau attempts to turn him back, Vakama has no problem beating him up and almost killing him. Despite Matau's efforts and heartfelt speech, Vakama couldn't bring himself to believe that Matau had gone through Character Development, and thought he was the same Jerk Ass he had been when they last met, so he felt no remorse after hurting him. That is, until Matau managed to prove to him then and there that he has truly changed.
Experienced at least twice by Kratos, when he murders his wife and child prior to the start of the series, and kills Athena while fighting Zeus at the climax of God of War 2.
In Jedi Knight: Mysteries of the Sith, this trope is the solution to the final duel of the game. As Mara Jade, you must holster your lightsaber and stop attacking Kyle in order for him to have to kill you unarmed if he wants to reach his goal, thus snapping out of it
Kingdom Hearts: Birth by Sleep has Terra experience this after nearly killing Master Eraqus while trying to protect Ven. Not at all helped by Master Xehanort finishing off the weakened Master Eraqus, who was also in the midst of a Heroic BSOD for raising his Keyblade against his students.
Drowtales: Ariel was heading toward the Anti-Hero/Anti-Villain grey area, but after accidentally killing a slave she became more of an Emotionless Girl, and more recently more of a Plucky Girl type. Because of this she ended up sparing an enemy and avoided directly killing someone else, though given the Crapsack World of the setting both of those choices may come back to bite her.
In Collar 6, Butterfly, aka Evita Kappel has one of these moments when she lashes out at her slave Gunther because he worries about her mental state. She immediately regrets it. Though she'd already been established as a villain, this also serves to back up Gunther's theory that her use of her husband's "black books" has started affecting her personality.
It was this sort of moment, when she recklessly injures one of her teammates in a drunken stupor, that convinces the heroic Swallowtail of the Global Guardians PBEM Universe to seek help for her alcoholism.
A strange mixture of in-universe and out-of-universe examples: in Twitch Plays Pokémon Crystal, the Mob accidentally releasing Togepi instead of LazerGator caused them to take stock of themselves, eventually leading to its new and distinct themes and plot.
Aang has a similar experience in Avatar: The Last Airbender . When Aang starts learning Firebending he proves his immense talent by turning a smoldering leaf into a ring of fire. Katara's hands get burned badly in the resulting blast, Sokka gets immensely pissed off at Aang for hurting his sister, and Aang, in shock, says he's sorry. Fortunately this is when Katara discovers she has (self) Healing Hands, but Aang ends up jumping to the conclusion that Firebending is strictly Bad Powers, Bad People, and resolves not to learn it. It would take two seasons for the cast to realize Firebending is not evil, and in fact life-affirming.
There's also an event that took years to build up. Katara inadvertently ends up learning "blood bending", controlling the water in someone's very bloodstream to turn them into a puppet. Realizing the horrible possibilities of this kind of talent, especially when she's forced to use it to stop the twisted old lady who taught her the technique, she vows never to use it again... Until Zuko leads her on the path of the one that led the raids that took all the water benders from the Southern Tribe and killed her mother. After years of sorrow and mourning, cue one Roaring Rampage of Revenge, that ends with her using blood bending to almost make the man responsible kill himself. Then she finds out, this guy is that admiral's REPLACEMENT. The actual admiral is now an old, retired man, leading a miserable life in his mom's house. She finally lets go of the hate; he'd rather she just kill him. Or his mom. "That's fair! Right?"
Happened on an episode of Static Shock where a kid being remorselessly tormented by bullies brings a gun to school. Just as the kid was about to give up the gun the bully's idiot friends tackled him which made him accidentally squeeze the trigger and he winds up shooting Richie, one of the few people who had tried to be nice to him. The kid suffers a massiveMy God, What Have I Done?.
A few episodes of Jimmy Two-Shoes had Heloise backing out of whatever evil she was doing because it was hurting Jimmy.
In an episode of Transformers Animated, an upgraded Prowl beats (clone) Starscream. Unfortunately there's a lot of collateral damage - including a torn down tree and broken bird eggs that he earlier made an effort to protect. Seeing these, Prowl realizes what a total asshole he's being.
My Little Pony: Friendship Is Magic: Discord has always had an utter disregard for the feelings of anyone else, and doesn't get snapped out of it until Fluttershy basically makes herself his Morality Pet by actually treating him as a friend. It's not until he realizes that his antics are driving her away that he finally grows some empathy.