Played strait in The Adventures Of Gyro, the Disney comic parody. In the story Gyro Gearloose - the nices person in Duckburg snaps for no reason, start kidnapping children, destroys characters property, burns down Scrooge's Money Bin, kills Magica De Spell by burning her alive and tires to butcher few more characters. Since Gyro is usualy the most plesant character nobody even suspects him of making such terrible things...
White Mage from 8-Bit Theater shows this after rebuilding a city and its inhabitants from destruction at various hands (although mostly Black Mage) only to have the city yet again annihilated at the hands of, apparently, Black Mage. Feeling distressed and angered (also mostly due to Black Mage, although Fighter had an unwitting hand in it), she changes her robes and begins bestowing her mediocre evil upon the world for a couple comics until changing back after a strange talk with Fighter.
One guest strip had Black Mage push her over the edge with one too many pickup lines, whereupon she began chasing him with a chainsaw.
A better example would be Fighter, the nicest, friendliest, ditziest character in the comic who has a very potent Berserk Button whenever his friends, especially Black Mage, are threatened. He went into an Unstoppable Rage when Black Mage was killed, fought a fire demon to a standstill after she killed Black Belt, and this comic.
Fighter: Also, I can block any attack and kill anything that bleeds. Hint.
Thief: Yes, it is a good thing. Leaving him to rot would be wrong.
Red Mage: So very wrong.
Fighter (holding up a sword): Kind of chainsaw wrong.
Thief: Okay, good thing we're all on the same page then!
While he is in no way a nice one, a case can be made for Sarda given that he used to be the Onion Kid whose life Black Mage repeatedly ruined and seeks revenge against the Light Warriors for doing nothing to stop Black Mage.
Dan Shive: There is nothing more dangerous in any universe being written by me than a nice character who's been pushed over the edge. And Grace is really, REALLY nice.
To not quite as potent an extent, Elliot. He's extremely nice and by default helpful. Of course, mocking Tedd or any of his friends changes things.
And now, we learn, Noah. Not only do his girly looks hide inhuman strength and speed, but his particular speech patterns are a deliberate focusing mechanism to keep control of an otherwise short temper.
Rikk, the leader of the Science Fiction Club in Fans!, is as nice, gentle and forgiving a man as you're ever likely to meet. However, during a storyline in Fans! which sees a bitter ex-boyfriend of Alisin's blackmail her into joining him, while simultaneously attempting to destroy the fan-club and replace it with a new, darker version - Rikk is finally pushed too far, and beats the ex-boyfriend to a pulp before someone stops him from going too far.
Piro from MegaTokyo is an example, as a group of perverted otaku voyeurs in a restaurant found out. This is more obvious in Piro's online alter-ego Piroko, and associated wallpapers/tshirt with a cute girl carrying a big gun, and the phrase "Ph34r t3h cute ones".
Ping from the same comic could be an example, as well. She's normally sweet, cute and caring, but get her angry and she proves to have the strength to uproot a street lamp and use it to beat a giant monster into submission.
An impact to the head can do it. When running from a crazed feral crab in the Beach Episode, he crashes headfirst into a sign, which triggers the aforementioned Angry Clown Mode, allowing him to handily demolish his tormentor.
Sam has a moment of this when blackmailing a ninja in this thisSam & Fuzzy strip.
Out of Ctrl+Alt+Del "Four Players" side story, Player Three, the yellow one, is the one to be careful of. Sure, he starts out as the Butt-Monkey of the other three players, each more sadistic than the next, but has been known to snap while put under extreme duress. Player Two fell victim to this, as Three grabbed him and slammed him against the wall until he ragdolled all while shouting, "STOP IT<". He then came to and tried to apologize for what he just did.
It's not as explicit now that element of her character seems to have been phased out, but Lorenda Soulstealer used to eat people who annoyed her, and she still acts enough like a demon to terrorize a group of papparazzis into leaving Jyrras alone even after he gets declared "This Year's Most Eligible Bachelor". Otherwise, though, she's a cheery, good-natured cow whose not offended in the slightest by comments on her chubbiness.
Gilgamesh Wulfenbach is a genius who never even bothered to build a death ray, and who will put up with massive annoyance to avoid killing people he neither likes nor respects, but when an enemy army really gets in his way, he smashes it in about five seconds.
Agatha starts out as a clueless student, then she starts taking levels in badass as she realizes her sparkiness and gets fighting lessons from Zeetha. She is still generally a nice one, but you don't want to make her angry. Killing her boyfriend is especially not a good idea.
Tarvek isn't exactly a "nice one", given his Chronic Backstabbing Disorder, but the one and only time we ever saw him be violent, it was a staged fight that both parties got a little too into. So this got quite the "Holy shit!" reaction from the fandom.
Liza from Tonja Steele is a good example, as she's usually a sweet-natured schoolteacher. Some of it is explained when we meet her parents◊.
Quentyn of Tales of the Questor gets his sword stolen, and in the course of retrieving it gets arrested multiple times, mugged, shot at, dropped through a roof, stripped naked in front of a crowd of toffs (and his childhood crush), and then violently beaten and tossed in a dumpster. He promptly girds up his loins and proceeds to utterly wreck not one, but TWO street gangs singlehandedly and literally pull an entire building down around their ears... and would have REALLY torn them a new one had the city guard not intervened.
The Raccoonan nation topped that in the companion strip, Quentyn Quinn, Space Ranger. In there, the horrific cannibal species, the Kvrk-Chk, made a public show of what they would do the Raccoonan race when they ate a number of them on camera, The Raccoonans retaliated the next day by destroying one of the heavier populated Kvrk-Chk solar systems and gave the race a very good reason to fear the Raccoonan people.
Faen from Drowtales. She's a rather shy, sensitive girl, who also happens to be a powerful Empath with less than perfect control over her powers. Strong fear/anger cause Power Incontinence and since she's a projective Empath, peopleget hurt.
Calvin/Freckle from Lackadaisy. Shy, scared, and gullible, but put him in a life-or-death situation and give him a gun...
Just about every one of The Dragon Doctors qualifies for this. Goofball wizard Sarin has a dark criminal past and has immense transformational powers when pressed. Kili the sweet and sensitive shaman can transform into a snarling werewolf due to her recent contraction of lycanthropy. Even their mostly-quiet leader Mori, a magical scientist, has combat options, and has invented a magic beam that can blow your clothes and weapons away.
Guy, from Mushroom Go. Guy is a literal shy guy that never speaks, happily dedicated to his work as the crew's nurse, cook, and housekeeper. Though afraid of hurting other people, he's proven himself a decent fighter—with scalpels and chainsaws.
Magic And Physics has SPDA who seems like a genuinely nice person (being maybe the only one in the strip), but all hints about his back story suggest something that could have easily cost lots of lives.
Or clock you in the jaw. Or, if you were the one who killed her, chainsaw you in half.
Dad Egbert goes from constantly baking desserts and writing letters of praise to his son to beating his way out of a prison on Derse and fighting huge monsters without batting an eye when he gets involved in the game. And as the only naturally-born human in the cast, he's also a true Badass Normal in a series where nothing is normal.
John totally fails to invoke this trope. Behold John getting angry:
Feferi Peixes is a cute, pink-themed princess who likes to take care of her many pets. She is also strong enough to pull whales through the ocean, and acts as a diplomat between trolls and Eldritch Abominations=.
As well as Åland, Sweden's boyfriend. Friendly, gentle, metrosexual... until Russia tries to invade and Åland lets loose all hell on Russia, hunting him down and even shooting out Russia's white flag of surrender. The comic being what it is, this is fully based on historical events.
Piffany is so full of diabetes-inducingly sweet virtue that it gives her Reality Warper powers, in addition to D&D cleric spells. It's very rare that she gets angry, but when she does, she's easily the most powerful character in the comic. And that's not counting that one storyline where she got holy Powered Armor...
Julie, from Our Little Adventure when an armored Minotaur kills Pauline, a member of her Nakama. Her bubbly, happy personality vanishes and is replaced by something violent and scary, complete with Creepy Monotone and Hidden Eyes. In the next page we see her rapier embedded in the Minotaur's skull.
Ashton in Winters In Lavelle spends half of the comic looking like a scared, lost puppy. However, when people start threatening him (or Kari), he manages to fight a random Gard, Xan, and Prince Edric.
The Freakangels are, in the words of Alice, kind of crap when you get right down to it. All of them have psychokinetic powers, and can do some serious damage all by themselves without lifting a finger. Out of the group, Arkady is the most gentle (except possibly Connor), and a Friend to All Living Things. But then Luke tries to Mind Rape some poor girl, and Arkady takes direct action.
Arkady:Do you know what that was Luke? That was exactly one second of my overdose experience when we were fifteen. If you ever do something like this again, I am going to give you five minutes of that.
In DOUBLE K (a Tengen Toppa Gurren Lagann fan comic), Simon is a sweet little kid who doesn't know what to do without Kamina giving him instructions. Unless you threaten Nia.
Simon:(talking to Kamina) I-I guess I just forgot how freaked out I was. I mean, I saw poor Nia out there, she was just scared out of her wits, bro, and-and then the suspect started moving in on her, all with that creepy look on his face, and then-then-I dunno, bro, I just-I just knew I had to do something.
Lero-Ro and Knag Horyang from Tower of God. One is always sublimely cheerful, righteous and nice, he is Ranker and therefore one of the most dangerous people of the Tower and sometimes demonstrates that. The other is a soft-spoken man of little words, but he too is incredibly righteous, and while he wishes wrongdoers best luck to survive him, he doesn't hold back.
Stand Still, Stay Silent: Tuuri is usually The Cutie, but mention her weight and she'll make you pay, even if you are a child. She also gets a manipulative younger sibling side identified by Mikkel when she directs it towards him.