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 hilarious manners. When he loses it (And you'll know it when he does), it's a sign that someone is going to get hurt.
* After Dende's Father and Brother are killed.*
Krillin: Breathe Gohan! You're a leaf. A leaf in a calm stream!
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.
While nice characters are few and far between in AJCO, the ones that are there should almost certainly be feared. Possibly because they're such a rare breed.
Kaja is motherly and forgiving, taking good care of her cheerful cherry-blossom town on the water with all the love one would expect. She's also a centuries-old former Knight Templar angel sworn to protect whoever asks for her help, and reveals her true form to A_J in a fit of anger when Egg is sentenced to death.
Mei is clumsy, friendly and permanently well-meaning, though as a zombie she can and possibly will tear her enemies limb from limb should they push her that far. She's practically indestrucible.
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. Hell, throw in Aquerna and Dragonrider while you're at it, as they've both been shown to bring the whoopass when needed.
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 extremelystupid. 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 Cloud Cuckoolander 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.
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.
Another is a sentient mass of water that takes the form of a woman. She was nice and friendly to male staff, but the experiments performed on her by them traumatized her so much, she finally had enough and lashed out at them which temporarily changed her object class to Euclid.
SCP-1609 used to be a perfectly harmless chair that would teleport behind you if you needed a seat. One botched decommission by woodchipper later, it's taken to teleporting its remaining splinters into people's lungs if it feels threatened.
When Dio kicks Danny in the Vaguely Recalling JoJo series, Jonathan mercilessly pummels Dio. Dio receives the same fate when he accidentally kicks Danny into the incinerator.
In Endless Saturdays, Kara, while perhaps the most quiet and normal of the three, gets annoyed enough by her siblings to try and ensure that they get permanently lost in the wilderness.
Skitter from Worm is in many ways the nicest of the Undersiders, a group of villains trying to take over Brockton Bay and will go to any lengths for those she takes under her protection, even for complete strangers. She's also the most powerful, dangerous and terrifying of the group, despite it containing darkness manipulators and a girl with an army of monster dogs. This is because she wants to use her power for good and has resolved to make full use of intimidation to scare people into submission rather than harming them and, failing that, is prepared to use brutal combat pragmatism to take down stronger threats that herself as quickly as possible.
In Super Powereds, hurting people Vince cares about is a very bad idea. Best exemplified at the end of Year 2, when he has a vision of a girl from his past dying and goes on a rampage, and near the end of Year 3 when Sasha dies and he goes to kill the man who did it.
In Dead West, Gervas, the narrator is a Kind Hearted Cat Lover, and a perfect gentleman. Cross him, and expect to be pulverized. The Porcelain Doctor is this combined with Beware Of The Quiet Ones; he is the sweetest, most polite, meek Bishounen in dainty glasses and always dressed sharply. But if you press a Berserk Button, prepare to die. The effect is more shocking since his aura will make you look at him like the most fragile and precious little thing in the world.