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.
Lampshaded in The Order of the Stick: "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, and they immediately backed down.
Strangely, Tarquin is actually quite forgiving of his children, even though they both flat out have tried to kill him. Even Evil Has Loved Ones and Moral Myopia.
Earlier in the strip, Vaarsuvius killed a young black dragon. Recently that dragon's mother came back to inform V that she intends to avenge herself upon the elf.
Both Vaarsuvius and the dragon were engaging in disproportionate, escalating retribution. V killed the dragon's son. The dragon was going to kill V's entire immediate family (spouse and 2 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.
He wasn't aiming for the kid, though. Sarda just moved out of the way.
As it turns out, his behavior towards 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.
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 Out at Home, Mim responds to Herman firing her (for spreading rumors about him) by vowing to murder him. She's not very good at it, though.
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
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 his 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.
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.
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.)
Who 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 completely 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.
Another notable incident is 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.
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.
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.
Gamzee finally loses it by seeing the video for ICP's "Miracles", which he denounces as blasphemy. Dave apparently thought he'd like it.
In Sonichu, a poorly disguised mashup of 4chan and Encyclopedia Dramatica make fun of the Author Avatar over the Internet. In response, he collapses their workplace, killing and injuring hundreds, and leaving hundreds more unaccounted for.
The first instance was of a "Jerkop" who attempted to kick him out of a mall. In response, he fires a Curse-ye-ha-me-ha at the guy, destroying his life and family in the process.
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.
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.
Angelo blows an underling to atoms for criticizing Brian's meeting plans in this comic of Our Little Adventure. Brian was a bit upset over it, but got over it quickly.
Fruit Incest features an ancient dragon capable of summoning giant stormclouds employed by the police to retrieve a stolen cash register.
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 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".
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.
