Beware the Nice Ones/Web Original

"* After Dende's Father and Brother are killed.* Krillin: Breathe Gohan! You're a leaf. A leaf in a calm stream! Gohan: 'FUCK THE STREAM!'
 * Gohan's adherence to this trope is parodied in Dragon Ball Z Abridged. When he is a shy, meek, nerdy kid with good manners, a lot of the characters in the series abuse him in hiarious manners. When he loses it (And you'll know it when he does), it's a sign that someone is going to get hurt.
 * Kicks Dodoria towards a house.*"


 * Codex of The Guild is shy and accommodating to the point of being an Extreme Doormat, but when she found out her boyfriend had dumped her for another man she set fire to his cello.
 * Lampshaded in Season 3 by Vork: "Codex, you of all people turning on me as well? Do I truly have the power to snap your gentle spirit?"
 * Zaboo is very creepy, but always sweet and giving, to the point of people wondering why Codex doesn't want to date him. And then he goes crazy on Valkyrie. Codex, whose plan it was to go after Valkyrie in the first place, says, "Ok, he's basically freaking both of us out now."
 * Clara was the other nice one of the guild. She was a sweet, if slightly out there, overgrown teenager who...tried to be a good mother. Probably. Then Vork didn't give her the orb, so she spent the next few days killing him nonstop.
 * The Saga of Tuck has several perfectly nice, ordinary characters who are all but trained covert operatives in intelligence, counterintelligence, black propaganda and sabotage.
 * In Red vs. Blue, Sheila is pretty nice and even-tempered, especially in comparison to the rest of the cast. She's also the installed AI in a tank.
 * Another AI example is Delta. Mostly emotionless, he's nonetheless shown subtle and very touching dedication to his partner York, even insisting on staying beside him into death. He's later revived, and partnered with South Dakota, who proceeds to try using him as bait to let herself escape. When South is captured, it's Delta that recommends that she not be given further opportunity for betrayals.
 * Caboose is another example. He started out as an idiot and only got dumber. However, he is by far the nicest member of the main cast; he cares more about making friends than fighting (unfortunately, when he does fight, generally he's a danger to his allies). However, when he worked himself up into a fury, aside from launching into a series of nonsensical one-liners ("Your toast has been burnt and no amount of scraping will remove the black parts!"), he defeated two armies of flag-obsessed zealots.
 * His nonsensical one-liners are actually helping him fight, because he's saying things that make him angry, prolonging his Unstoppable Rage. You Fight Like a Cow probably makes the slaughter easier for him.
 * Caboose is also Nigh Invulnerable, equipped with a Super Strength unit, man enough to be in a temporary relationship with a tank, and just wants to keep his best friend Church safe.
 * Don't forget that his Super-Powered Evil Side is caused by him imprinting an evil AI, (also a MacGuffin)
 * Doc was also a case while he was stuck with O'Malley for a while.
 * The flashback subplot in Season 9 shows that Washington was pretty meek for a Freelancer, contrasting with his established personality and actions.
 * Among many in the FFRP forum The Town/Acronymia, Pesty. A cute combination of Genki and Cloud Cuckoolander catgirl. She adores her boyfriend Grant, their adopted daughter Pixie, and lots of other people. When Grant's crazy and evil side tries to take him over, she snaps him out of it by bitch-slapping him and shouting in his face. In a future-possibility-what-if plot thread, the kidnapper of Pesty's grandson earns him an utterly nightmarish end.
 * In the Whateley Universe, Jade Sinclair (Generator) qualifies. Normally the cute, shy, wacky 'little sister' figure, when she's pushed too hard one day, and then attacked by Bloodwolf (an avatar of the werewolf spirit) and two fellow Ultraviolents, she snaps. Bloodwolf ends up nailed to a tree. With railroad spikes.
 * Lately she seems to be actively considering a potential career as the Psycho Babysitter From Hell -- perfectly harmless to her future charges, but woe to whoever thinks to threaten them.
 * It bears mentioning that, of those among her circle of friends who have killed other people (which include a terrifyingly powerful mage, a girl who wields a sword that can cut through anything, a soul-eating demon and Tennyo The Motherfucking Destroyer), Jade's kill count exceeds the rest combined by a factor of four. At the very least.
 * Speaking of which, Tennyo herself is a very kind, if somewhat somber, person who spends a lot of time worrying about accidentally hurting others with her powers. Considering how powerful she is, she has good reason to worry, so it is generally considered a very bad idea to give her a reason not to care about this.
 * In fact, let's put the entirety of Team Kimba, plus Carmilla, on the list.
 * Chakats are usually nice to a fault -- empathic, social, family-oriented, and almost too good to be true except that they normally actually are; their designers did a good job. Don't, however, threaten one's cubs or even hir best friends and family will have trouble keeping hir from shredding you on the spot...
 * Alfred, the massive construction worker from Darwin's Soldiers, is explicitly described as not "the kind of person to pick fights". And it takes a lot to provoke him into physical action. But anger him enough and you'll see why angering someone with Super Strength is extremely stupid. His most extreme case was when he got so angry that he threatened to dismember someone and beat them to death with their own limbs. And if Dr. Zanasiu hadn't calmed him down, there was no doubt he would have done so.
 * The eponymous doctor in Dr. Horrible's Sing-Along Blog. Despite being a supervillain, he wouldn't dare think about murder or fight in a park with kids. Then Captain Hammer talked about doing Penny...
 * Awkward gives us Lester, a wonderfully amiable and somewhat put-upon hero who will put up with quite a lot -- until he actually seems to be making progress with Steph, and his live-in Stalker with a Crush Alex interferes and refuses to be ignored.
 * Survival of the Fittest is perhaps one of the most perfect environments for turning nice guys/girls into murderous psychopaths. Perhaps one of the most jarring recent examples being Liam Brooks in v4, originally a rather quiet character with a love for gardening and a phobia of blood, but after the death of his girlfriend he dove headfirst onto the crazy train, putting together a makeshift garden with said girlfriend at the center while he proceeded to brutally kill three people and injure a third trying to add more bodies to it.
 * A lesser example in v4 would be Maria Graham, a happy-go-lucky Cloudcuckoolander who was rendered almost catatonic for days after being beaten half to death and accidentally setting someone on fire trying to escape. However, after watching Maxwell Lombardi kill the boy she was in love with and her best friend, one after the other, and then try to kill her while mocking the former, she finally snapped hard and the two ended up viciously beating on each other in a burning building.
 * Matt of Two Best Friends Play is usually the calmer of the two friends, preferring Lighter and Softer games. But when he plays Deus Ex Human Revolution, after some initial reluctance, Matt goes nuts with the Video Game Cruelty Potential, stabbing and punching all the NPCs he can find.
 * The Gungan Council. Io Akima seems to be one of the most lovable, playful persons around. That is, until Adira smashed her datapad, which was a gift from a friend that had died. Cue Io going from wanting to tease Adira to outright attempting to murder her.
 * The title character of Erikas New Perfume is normally a loving, soft spoken Shrinking Violet, but when Big Sister Bully Marie tries to she snaps and lays into her with an epic combination "The Reason You Suck" Speech / World of Cardboard Speech that leaves her flabbergasted. In a wet diaper, no less.
 * On an episode of Dolan Life Mysteries, If We Stacked Everyone On Earth, Could We Reach The Moon?, Melissa, who's normally calm, beats up  Hellbent for being a jerk.