At the conclusion of the Mechakara arc in Atop the Fourth Wall, Dr. Insano captures the crippled Mechakara and tortures him with a magnet. Why? The robot made Insano cut his hair.
Speaking of Mechakara, the only reason he murdered his universe's version of Linkara and skinned him so he could use magical devices and started a Robot War to Kill All Humans...was because he thought Linkara's voice was annoying and he wanted him to shut up.
Ice the Frosty Cat utterly destroyed Ranger in the most humiliating way he possibly could in game 10 of Comic Fury Werewolf... for something Ranger did in Game 6—catch that Ice was a werewolf. Keep in mind, by that point, Ranger had died in game 6, so Ice didn't lose because of Ranger.
The SWAG leader of Demo Reel assumes that kidnapping someone at gunpoint and leaving them to starve to death slowly in an abandoned patch of woods is justifiable punishment for his territory getting threatened.
In Dragon Ball Z Abridged when Frieza asks Gohan and Krillin what planet they come from Krillin almost tells him out of fear but Gohan stops him... only for Dendei to do it out of revenge for Krillin constantly calling him Little Green.
Doctor XX (she pronounces is "Doctor Double-X") from the Global Guardians PBEM Universe is a Mad Scientist supervillain whose only real goal is the complete and utter eradication of all male life on the planet (male humans, male dogs, male chickens, male fish... doesn't matter; if its male, it has to go). Why does she do this? Because she once lived with a man who was a verbally abusive dick.
"If you ever, ever break my orders again... as God as my witness I will devote whatever time I have out of jail to making your lives living Hells. I will break your knees. I will beat up your wives and kids. I will make fun of your mothers and kill your fucking pets in front of your eyes. My wrath will be as extensive as it will be disproportionate, because I have no. Fucking. Sense. Of perspective. Do you idiots hear me?".
In the Whateley Universe, Overclock plotted to cause Tennyo to kill all her friends and be driven insane. Her crime? She ate the last of his favorite cereal in the cafeteria one morning.
Martial arts is one of the standard Physical Education classes at Whateley Academy. Counterpoint (power mimic with awesome powers and centuries of experience as a fighter) spars against Jobe (ordinary baseline powers but a genius with bio-weapons). Jobe wins by fighting dirty, but what happens on the mat stays onthe mat, right? Not with the god of disproportionate retribution, Counterpoint. He pummels Jobe mercilessly. Does Jobe tell the authorities? No, he retaliates with his own disproportionate retribution by dropping a biowarfare weapon in their food. When Counterpoint recovers...
Strega's daughter is kidnapped by a secret society based in France. She then bombs the headquarters, captures Paris and holds the city hostage, and promises a invasion of Europe greater than World War II.
In Worm the supervillain Accord's Super OCD is so strong that he wants to have a character executed for intruding on his meeting unannounced.
Oh hey, you put a regular axe through my car? Okay. I'll just spend my entire savings putting a giant axe through yourcar.
There's a video floating around of a girl who's supposedly pissed off that her boyfriend plays so many video games, so she takes his Xbox 360 outside and tries to bust it up with a golf club-tries being the operative word, because she has no idea how to swing the thing, so it really amounts more to gentle taps; her initial careless toss to the ground does more damage. Then comes the follow-up video, where the boyfriend objects to her mistreatment of his Xbox, so he takes her laptop and shows her how to really swing, demolishing the computer's screen in one blow.