In Christian Humber Reloadednote A crossover involving too many fictional works to list here, the main character goes after and kills a hunter who set a trap that he stepped into, even though the trap broke on his leg. He later kills his friend Soku, her entire family and all her friends because she turned him in to the police.
In "Days of Futures Past What Does That Mean It Kinda Sounds Sexy" Kurama tells Robin That someone once insulted ramen in Naruto's presence so Naruto retaliated by tying them up to a Prius and throwing the person off a bridge. Kurama doesn't say if the Prius was used to suspend the person off the bridge and dangle upside down or if Naruto threw the car afterward to help sink that person to the water bed bellow. Most likely the latter due to the fact that Naruto doesn't like Prius vehicles for some reason.
Death Note Equestria: Twilight, as Kira, kills criminals of various caliber — her main targets are Karma Houdinis and those she considers unrepentant — so a pony with a life-long history of minor offences might also be offed. Second Kira (Fluttershy), meanwhile, openly kills those who speak against Kira.
The Prayer Warriors often engage in this, particularly by deciding that anyone who disagrees with them deserves to die. One example is Idiosy, for whom hacking into the author's account is cited as an in-universe reason for having Rachael kill him and burn down his house, killing all his wives and guards.
In Renegade, the Global Defense Initative uses this as a basic part of their military doctrine. If someone fucks with a GDI colony, their response is to utterly destroy whoever attacked them with overwhelming force. This doctrine lead to them obliterating the pirate bases at Torfan from orbit, and results in full-scale war with the Batarian Hegemony due to them supporting attacks on human colonies. One Citadel politician accuses GDI of "making an example" out of the batarians, which GDI pretty much agrees with.
Murder is shown as a very rare and serious crime in Equestria, and the punishment for the guilty is banishment... to the Moonor the Sun. After hearing this, Phoenix immediately takes it as going too far.
Then there's Trixie, who took the job as the prosecutor as an opportunity to get revenge on Twilight and the mane 6 in general for revealing her as a fraud. Although it's implied later that that is not Trixie's main reason for wanting to back at Twilight, as indicated by the presence of black psyche-locks when asked about it.
Most of the main cast in Dragon Age The Crown Of Thorns, and some others, feel that the DN protagonist lashes out at Isolde a bit too strongly, although, true enough, she did outright accuse him that he doesn't mind killing children. Basically, he makes her cry with "The Reason You Suck" Speech and even deliberately makes her think he is going to/did kill Connor. Fortunately, he'd only gone upstairs to give the kid a hug and a So Proud of You, plus talk to the demon possessing him face to face.
When Connor is finally exorcized, the DN gets quite angry at Alim's impersonation of the Grim Reaper and, instead of congratulating him and whatnot, drags the mage over to give him a What the Hell, Hero?? rant in private. Alim counters it by calling him out on how he reacted too harshly, showing off his own Guile Hero traits by perceiving one of Raonar's biggest regrets.
In The New Retcons, Gavin Caine opposed Tracey's campaign to investigate whether Millborough being built on a former nuclear test site had an adverse effect on the population. Why? To get back at his son Anthony through Tracey and her husband Gordon, first for Anthony working for them instead of him, then for opposing his expansion plans for being outdated.
Kortney Krelbutz wanted revenge on Moira and Elly, the former for firing her, the latter for not hiring her back. (Never mind that she was busted stealing.) She steals Elly's husband John from her (not that she ever knew, given her untimely death), inherited his millions when he died, then used that to buy out the store she was fired from and fire Moira in turn, then bought out most of Millborough to have it absorbed into Greater Scarborough.
Remember in Harry Potter and the Goblet of Fire when Ron and Harry were having an argument, because Ron was feeling Overshadowed by Awesome and he felt Harry was hiding something from him? Well, Rose Potter of The Girl Who Lived responded to this by throwing Ron around the room and carving words into his forehead.
In My Immortal, "Samoro" (allegedly James Potter) and "Lucian" (allegedly Lucius Malfoy) are in a band together. Lucian plays a few wrong notes during a concert, and Samoro responds by attempting to shoot off Lucian's arm. But it was later revealed that Snap(e) used Demonic Possession on Samoro to make him do the above, meaning this trope is subverted.
Not to mention when Ebony's friend Willow becomes a 'prep', she is then murdered and her body is dumped in the woods. And is raped by 'Loopin', because he's a 'necphilak'.
There's also the time Ebony and Satan start to have sex with each other in a movie theater while the movie is playing. When the woman seated behind them complains, Ebony retaliates by killing her and sucking all her blood. (Well, she is a vampire.)
Hitman Miami: In one of the chapters, it is mentioned that should 47 even trip off a metal detector, an army of guards will shoot him to death, "as is standard procedure in the event someone sets off a metal detector, anywhere, in the entire world." (This parodies the All Crimes Are Equal trope from the original games.)
A rather brutal example occurs in The Kids Who Cried 'Ouch', an answer fanfic to the episode "The Boy Who Cried Gorilla." In the fanfic, Ryan, Connie, and Renee get their "just desserts" for their initial treatment of Richie when he first sees the gorilla. Among other things, Ryan gets beaten to the point that he suffers a drug addiction that causes his grades to fall to flunking levels, Connie gets a knife up the vagina rendering her sterile, and Renee winds up repeatedly getting raped to the point that she winds up pregnant and ends up suffering a miscarriage.
Hey kids, is the nerd asking questions that the teacher holds off recess to answer? That's not cool! Let's trick her into going into a forest full of hungry monsters to an ancient and quite possibly cursed ruin where she'll get mind raped by memories she didn't know she had! Sound harsh? In Past Sins Diamond Tiara and Silver Spoon did it! Although, to be fair, they didn't know that the mind rape thing would happen. The monsters, on the other hand, are common knowledge.
One conclusion to the My Little Pony: Friendship is MagicDark Fic "Pattycakes"note which, to be fair, is intended as parody ends with Fluttershy singing to Rainbow Dash about how Dashnote who does have a bad habit of saying things without thinking first will never hurt anyone's feelings again now that she's been mentally regressed to an infant. The other conclusion has Scootaloo fail to grasp Fluttershy's philosophy after going through the series of (twisted) tests, and if she fails the final test by drinking water instead of milk, she's regressed even further than Dash (Dash had Baby Talk, Scoot loses the power of speech entirely), with no hope of ever recovering. (For your daily humour-so-dark-light-cannot-escape-its-surface needs, her cutie mark takes the form of a baby's pacifier, since she's not exactly going to have the chance to demonstrate any other talents, now is she?)
When Diamond Tiara went through her mother's jewelry and "borrowed" a tiara (earning her cutie mark in the process), her mother didn't take it well. And by that I mean she tried to strangle her own daughter to death.
Let's not forget the Dark World arc, where Nightmare Eclipse/Paradox, formerly Twilight Sparkle, has been rewinding time simply to continue to make Discord suffer. An interesting example, because Discord really did enough to her to warrant a truly horrific punishment, Paradox managed to go so far, her revenge went into this trope despite how much Discord did to her.
In Sophistication And Betrayal, Cashmere wants to ruin Rarity and destroy her business, simply because the latter declined her affections.
My Little Unicorn: Titan got his face disfigured in his past and got shunned by everyone for his hideous looks afterwards. Years later, he kills his entire species and starts to destroy parts of the multiverse for no other reason, than that it is fun.
The author wrote the entire fanfic, which insults bronies as being losers who dare to believe in positive things, because one single brony made fun of him.
In the sequel he makes Cadance suffer, just because he does not like the fact, that in the show she got happily married
Ace Ray gets sent to an insane asylum and essentially brainwashed into liking Star Fleet, for the crime of... saying mean things about them.
Cadance Of Cloudsdale: Lady Prisimia was banished from Equestria because her boyfriend grew bored of her and started spreading Malicious Slander so intense that Celestia herself exiled her. Actually a Subversion, as Cadance notes how Prismia was under the spell of an evil amulet that amplified the negative traits of her personality and altered her perceptions of reality. It was far more likely that Prismia grew increasingly scathing and hostile until a real incident happened, and she chose self-exile than stay any longer in Canterlot.
Same with Prismia's reasons for stealing love from everyone in Reduit. Because of her nasty breakup with her coltfriend, she decided to take love from these people "because they didn't deserve to be happy" while she was miserable.
Invoked in Escape From The Hokage's Hat. The council want to banish Naruto, want Sasuke's prison sentence forgotten and were smart enough to get the Daimyo to agree with any of their decisions. Tsunade's response? Shove Sasuke in one of the darkest ANBU cells being kept barely alive, setting up the collapse of Konoha's economy, taking money from the accounts of every clan that ever wronged Naruto and finally making Jiraiya (temporary) Sixth Hokage while she leaves with Shizune, Hinata and Naruto for a six month training trip. When she explains why she did it to Naruto she has this to say:
"I decided to teach those meddling old fools a lesson in respect. If the reason I came back to the village is kicked out, I am SO joining in the fun!"
In New World, Maddie smacks Ashton in the face. And he proceeds to savagely beat her. Keep in mind that the reason she hit him in the first place was because he was tring to rape her.
In Latias' Journey, the Ghost King proposes to Latias after sending her several gifts. She refuses his advances, so he kills her family, and then destoys her home, Altomare, along with everyone in it. Judging by the city it is based on, he killed upwards of 800,000 people. Strangely enough, she still falls in love with him.
In the Poké Wars series, Ho-Oh sees genocide as a fitting punishment for humanity's "enslavement" of Pokémon.
In Plasma's Folly, Kelvin assembles an army of fifty kids who can turn into Legendary Pokémon to outright obliterate Team Plasma because his girlfriend was made into Genesect.
Inverted in With Strings Attached. Because the four are Actual Pacifists, when they are treated very badly (even tortured in a way) by the Uneasy Alliance in Ehndris—at one point, part of George's soul is taken from him—and have a chance to get some of their own back, they just slap people around for a few seconds. The worst that happens is a few people get slammed into a wall or hosed down, and George does “cat tornado” on Brox's head.
It's even more disproportionate when you realize that George did nothing else during the final battle except try not to get hurt, and Paul did nothing except absorb damage and hold Kerrun upside-down and tap her head on the floor to keep her from concentrating.
In Joker's Wild, the Joker kidnaps Beast Boy and puts him through horrific Cold-Blooded Torture for weeks on end. Why? Because an action figure of Beast boy just happened to be programmed to say "Dude, I'm the King of Jokers!"
In Gensokyo 20XXII Ran was scalded with some kind of liquid (hinted at to be either acid or very hot water) for snooping through papers and was left with severe burns, resulting scars after she healed. That is a pure testament to how brutal their captors are.
One tiny piece of artwork describes Makari, Warlord Ghazghkull mag Uruk Thraka's banner carrier, as his one and only genuine friend. You do happen to remember Ghazghkull's crusade to unite the whole Orkoid race and wipe out Humanity, before clearing out everyone else? That's because he hates them for murdering his lucky Gretchin. In the story written for the artwork, Ghazghkull orders his WAAAGH! to occasionally return to the world where Makari's grave is buried. He then goes to the grave alone and just talks to Makari about his WAAAGH's process. So far, he is succeeding.