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.
Max of Code Name: Hunter fills this trope pretty well. Sure that he is responsible and mild-manered (and a little shy), but just don't you forget that Max has years of trainning in swordfigtht and experience in beat up warlocks and magical creatures. So, if you leave him no choice but fight (or pushing him too far), Max will Kick your assvery painfully.
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.
Nice people in the Walkyverse usually get this. Joyce is a prime example. So is Walky himself, for that matter. Not to mention Amber... this list could go on for a bit.
Joyce: "This is for the thousands of little comments that are hammered into me every day! "Stop giggling, Joyce!" "Put down your dumb toys, Joyce!" You can only take so much... Until you snap."
Lemmy in Fanboys is a genuinely nice guy, but woe be unto anybody that says Nintendo is a kiddy system.
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 and Fuzzy strip.
Out of Ctrl+Alt+Del's "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.
For that matter, Dan. While normally a nice guy with a vague career of adventuring behind him, recent eventshaveshown just how dangerous he really is after being pushed over the edge. It's not pretty.
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◊.
Elan is mostly naive, sweet-natured but ditzyCloudcuckoolander. Elan in strip #594, after Smug Snake Kubota poisons his lackey Therkla (who was in the midst of a Heel Face Turn) and she dies in Elan's arms; Unstoppable Rage-driven Bad Ass. It's only Kubota's swift-but-slimy opportunism that prevents him from getting a severely lethal ass-kicking at Elan's hands, and even then he still gets reduced to dust by V.
The the Monster in the Dark has not yet gone past the breaking point, strip #475 suggests that making the monster angry is an idea so stupid that it borders on Too Dumb to Live.
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.
Plant Woman. It's one thing if you've only built *two* machines of death and destruction. The first law of robotics is thrown out the window completely when you announce the third.
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 Cats. 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.
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...
The Oswald Chronicles has the eponymous Oswald, a mild-mannered and polite gentlemouse. He prefers to talk things out instead of resorting to violence. Hurt his friends, however, and...
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 on a good day, she's a slightly unstable Cloud Cuckoolander, and a Mad Oracle to boot, but 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.
The Coffee Shop Lady from Sinfest is the nicest, sweetest granny archetype you could imagine, dispensing kindness, coffee and donuts to all, even distraught Devil Girls but don't try to push her around. Also counts of course as a case of Never Mess with Granny.
In DOUBLE K (a 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.
Kieri Suizahn, an angel from Slightly Damned, is usually very shy and soft spoken. A history of being abused by her parents and peers does make her rather sensitive to bullying, even from other angels.
Bears noting that that is one of many instances. Mega Man is the nicest guy you could meet, but if you threaten any innocent people, or worse, actually kill people, he will fuck you up.