In Ansem Retort, Darth Maul attacked Marluxia, complete with Force Lightning...all because Marluxia said mango-strawberry was a gay flavor.
And one time Aerith attacked an airline pilot just because the in-flight movie was The Devil Wears Prada. Axel declared it the best honeymoon ever.
New and insanely disproportionate example: Axel and Zexion throw Riku in a volcano and nuke said volcano in an attempt to recreate the Scientology origin, trick him into creating mustard gas that burns off his flesh, and beat him with a sack of failed ideas for retribution. What did Riku do? He slapped them. And this was actually a good thing. (Slapping them helped restore their memories of the original timeline.)
Whom are we kidding? Ansem Retort loves this. At her bachelorette party, Aerith gets so drunk she forgets that she's marrying Axel and runs off with another guy. The response? Axel and Zexion HIJACK AND CRASH A PLANE INTO HIS HOUSE. A PLANE. To be fair, they had already hijacked the plane earlier in the evening, before learning of the above event. Besides, they had to do SOMETHING with the plane.
Axel hanging Riku from a tree with a lei, in response to Riku making the "whipped" sound ("wapish!") after seeing Aerith tell Axel to take care of their luggage. In Axel's defense, he DID warn everyone that he would hang them with a flower necklace if they made the whip noise.
Also, for having a not-yet-confirmed relationship long distance with Sandy, Mike was brutally beaten by Paulo and the Female Student Body for his friendship and UST with Lucy in chapter 20. Twice. Played for Laughs.
Mab from Dan and Mab's Furry Adventures was exiled from the fae kingdom after her friend Nutmeg, as the acting queen, passed a law that no fae may have a fluffier tail than the queen (Fae laws are rather whimsical), and Mab refused to de-fluff for anyone. Nutmeg's husband, Albanion, has a tendency to swear vendettas on anyone, including a pair of girl scouts (for not having the peanut butter cookies). Fortunately, as fae have a short attention span and a long life span, the foolish mortal that is a target of said vendetta usually dies of old age before the Fae finally comes up with the perfect revenge.
At least Mab eventually got over it. Granted, she did things like run Nutmeg through the torso with a spear before she got over it, but to Fae that's barely a minor inconvenience.
It is also implied that a great number of these things are examples of kidding on the square; while they really do mean it, they also don't take it very seriously. Additionally, in some cases, such as Albanion, it is heavily implied that these things are done with ulterior motives - in the case of Jyrras, it is outright stated that the bangles that he places on the inventor to signify his vendetta is basically a "do not touch" sign as far as other creatures are concerned, as no one wants to annoy a fae who has spent years, possibly decades or centuries plotting revenge. The fact that many people die of old age (as opposed to other things) before the vendetta is acted upon may well indicate that such "vendettas" are often used for such a reason.
This is how La Familia protect the secret of the Ring Drive. Mega Corp. steals a ship? Send half an armada to destroy their shipyards until they give it back. They try it again later? Raze their HQ. Two officers lock themselves inside the drive? Order the captain of the ship to fly it into a star. Thieves steal the plans? Eject them into space. Naked.
In Fans!, Robert's start of darkness is when... Alisin killed his pet fish. Keith, whose start of darkness was being physically and mentally violated by Alisin, is rather unimpressed. It is noted that Robert's real problem is an unwillingness to confront reality, to the point that he would value a fish so highly due to his inability to connect with anything else, but STILL.
The Fox Sister: After Yun Hee harasses the Kumiho by plastering Pujok to ward off evil spirits all over the place, she retaliates by messily killing a group of three young men right at Yun Hee's front porch.
Fruit Incest features an ancient dragon capable of summoning giant stormclouds employed by the police to retrieve a stolen cash register.
In General Protection Fault, Trudy and Trent apply for a job, and agree to let Trudy apply alone. Trent takes the application and fraudulently gets the job, but Trudy exposes him and gets it herself, while arranging for him to be arrested on trumped-up charges. Trent's own Disproportionate Retribution comes when he tries to frame Trudy for dropping a safe on Dwayne and expose her other safe droppings. Neither one is a very pleasant person, to say the least.
In Homestuck, a dispute over a harlequin costume ends in blood.
After Aradia haunts her with creepy but harmless spirits for crippling Tavros, Vriska mindcontrols Sollux to KILL her.
In all fairness, we don't actually know if the ghosts were harmless.
The crippling of Tavros was itself an example since Tavros's "sin" was attempting to quit a FLARP game.
Gamzee finally loses it after seeing the video for ICP's "Miracles", which he denounces as blasphemy. Dave apparently thought he'd like it.
In Act 6, the Alpha session Draconian Dignitary has - inspired by Jane's father - made wearing suits mandatory and failing to do so punishable by death.
The Kenny Chronicles: "Kenny vs. Funky part 2" involves Funky kidnapping Kenny, stealing his collar (with his money and ID on it) and dropping him on the mainland; the reason, Kenny drew a picture of Funky's head on a pike. Later, Kenny's eye is temporarily injured by a (non-anthropomorphic) cat and Funky spreads rumors that he was responsible, so Kenny tries to kill him.
Though the reason why Kenny drew that picture in the first place is kind of an inversion, Funky took a video of him having sex in a dumpster.
Vin Vulpen has a tendency to do this to Rudy in Kevin & Kell, first contacting the Institute for Species Purity to report his teammates' unconventional lifestyles, resulting in the team getting death threats, and later pours prey pheromones on Rudy during a night hunting event, putting him in danger of being eaten by his own teammates, all largely the result of jealousy over Rudy taking his star status on the team and losing Fiona and Rhonda. After Angelique fires him from the Herd Thinners lab for making several mistakes, including blowing off her fake rat ears, he tries to frame Angelique and R.L. for fraud.
minus: The title character's attempts at Laser-Guided Karma sometimes turn out to be this, which is perhaps justified as being a child she doesn't have a well-developed sense of moral responsibility when it comes to her reality-warping powers. This strip being a prime example, in which a hapless balloon-seller whose product is prone to popping ends up being turned into a balloon himself... which eventually pops. His only crimes being selling the odd defective product and getting annoyed...
"The Linear Guild is practically synonymous with taking disproportionate revenge over quasi-imagined slights."
Nale gets this from his father Tarquin. He gets a pair of bounty hunters sentenced to die in gladiatorial combat by screwing around with their paperwork. Although he did it because they tried to extort a large sum of gold out of the him (an extra 42,000 gold pieces, which amounts to 840 pounds of gold), they only did it because they were trying to keep up with the barrage of Star Wars references that just occurred. Their "thermal detonator" was a can of soup, they immediately backed down when it was pointed out, and they flat out admitted that's the only reason they did it.
Tarquin is forgiving of his children, even though they both flat out have tried to kill him. Even Evil Has Loved Ones and Moral Myopia. Eventually, Tarquin gives Nale some proportionate retribution. After a lifetime of constantly undermining him and just after killing his best friend in an unrecoverable way, Nale tells his father in no uncertain terms that he doesn't want his favor. After verifying that is truly what he wants, Tarquin stabs Nale in the chest.
Escalation of responses. V killed the dragon's son. The dragon was going to kill V's entire immediate family (spouse and two children) before binding the children's souls to make resurrecting them or safe passage to the afterlife impossible, and leave that plane so V could never save the childrens' souls or get revenge. V then killed the dragon, every dragon directly related to her, and every dragon related to those. By V's own figures, that was roughly one-quarter of the black dragons alive at the time.
Haley and Crystal's deadly feud started over a catty conversation where they each insulted the other's hair and boots respectively.
When Roy and Xykon first meet, Xykon comments on Roy's motivation for killing him;
So, let me get this straight. Your father spent his entire life looking for me over a largely inconsequential killing?...And he was too much of a loser to get the job done before croaking, so now you, Loser Jr., have taken up the task?...Y'know, I've destroyed entire towns and the most I got from the surviving families were a few snarky comments. You, sir, have a serious problem with overreaction.
Schlock Mercenary plays with this a lot, although it's often subverted by the fact that in military operations, "overwhelming force" is not at all an unreasonable place to start from. One in-universe saying is "There is no overkill. There is only 'open fire' and 'I need to reload.'" A few of the straighter examples of the trope:
A planetary legislature hasn't allocated funds to replace their orbital defenses, a couple months after god-like AI Petey defended them from attempted orbital bombardment. Petey finds this irresponsible, and exiles them to the Andromeda galaxy.
The Obenn at one point decide to grind one of the protagonists into sausage. Because he was insufficiently polite during his interrogation.
Part of the reason people got so upset over Trike Girl's servicing Charlie Brown with a restraining order over his obsessing The Little Red Haired Girl is because Charlie Brown is a prepubescent kid with no history of violence and doesn't know better, so it's the kind of thing that would be best cleared up by having her parents/the teacher talk with him instead of potentially giving a little kid a criminal record.
Not to mention that she is really only talked about seen in school or in areas like a park, so saying he is obsessed over her is also disproportionate.
Either way, intentionally or not, Trike Girl pretty much embodies this trope, causing trouble even or especially for entities that realistically have nothing whatsoever to do with her sole ostensible character trait, and constantly deriding women for their minor lifestyle choices with essentially no provocation, essentially attempting to brainwash them into paranoid clones of herself for offenses like dressing "too provocatively" or "not fighting the patriarchy hard enough".
Bun-Bun from Sluggy Freelance hates being bothered by telemarketers; the kind of hate that makes someone go on a Kill Bill style Roaring Rampage of Revenge. He'll respond to any slight with extreme savagery, though; telemarketers are just at the top of the list.
Then there's the time Riff sics Bun-Bun on Harmless Villain Minion Master because "he questioned my scienceness."
When chipmunks steal tomatoes from Torg's garden, he and Riff plot "Operation Orange Tomato", which involves plans to exterminate all chipmunks.
Chris-Chan inflicts a horrible curse on the Jerkhief that results in the loss of the his family, home, and everything he owns—for arresting him over... something. It was never properly explained.
In Sonichu 10, Author Avatar Chris and Sonichu attack the 4-cent_garbage.com Tower with The Power of Rock which causes the building to collapse, killing 100 of its workers and injuring another 150. Chris-Chan's following dialogue makes it clear that he thinks they deserved it... for being trolls.
A guy accidentally calls Family Guy's Meg Griffin a man. Chris-Chan gets upset. The guy accidentally calls him a woman. He punches the man in the face.
Based on rather flimsy evidence, they linked Alec, Evan, Sean, and Mao to the murder of Simonla Rosechu. So what does Author Avatar Chris do? Have a show trial with an obviously biased judge, order the four to be executed and then kill them onscreen in the most brutal ways possible.
In Superosity, Jork's rival Hayseed faked the death of Jork's wife and wiped her memory...because Jork once insulted Hayseed's favorite tie. A tie which read "Jork Sucks".
TwoKinds: Trace's wife dies from a poisoned arrow, he reacts to this by taking over his mage order, turning them from Jedi-like peacekeepers into an army of Knight Templars and launching a campaign of genocide on the race of the guy who shot her. Granted, he did do most of this under the influence of the dark mana he used to try and resurrect his wife, but still...
His current girlfriend just got a minor flesh wound. His reaction? KILL EVERYTHING
Van Von Hunter features a character who dedicates himself to this. His name is Vengeance Joe, and he is on a quest to slay Von Hunter for not introducing himself.
In thisVG Cats strip, Leo spends the first half of the comic making lame jokes. Aeris' response is to go back in time and abort him out of existence.
He wasn't aiming for the kid, though. Sarda just moved out of the way.
As it turns out, his behavior toward the Light Warriors (except for Fighter) is entirely justified: He's the Onion Kid.
Still falls under this trope though. The Light Warriors never knew the harm they had caused the Onion Kid, and his idea of just revenge is to allow them to slaughter, torment, and torture their way up to high levels, just so he can have the fun of lowering them again. In some ways you can say he's more guilty of the atrocities than the Light Warriors, since he knew what they were doing and allowed it, while some of them were caused unknowingly by the Light Warriors.