Downfall is chock full of (relatively) nice, non-combative people. When they decide to get busy it tends to get ugly, mostly because the non-aggressive characters are usually the more powerful ones.
The Wizard In The Shadows has Harry, after he opens up a little. Nice, friendly, caring and slightly but harmlessly mad. Until you hurt someone he cares about. Then you are reminded that he earned his status as The Dreaded for the forces of evil, and that he is the most powerful mortal wizard alive. When Harry gets angry, craters the size of soccer pitches are left behind and the Nazgul make for the hills.
Stray contains an incident where this trope is applied to Otacon, of all people. It turns out that the quirky, tenderhearted, non-action anime geek can be dangerously resourceful when backed into a corner.
In the Chip N Dale Rescue Rangers fan comic Of Mice And Mayhem Gadget, usually the epitome of kindness, attacks the men who were after her when Chip gets shot trying to protect her.
Latias from Latias' Journey. She's depicted mostly as a gentle, somewhat timid and fearful Pokemon. That changes big time in Chapter 5. After watching a bunch of evil bird Pokemon butcher a bunch of baby Flying and Bug types in Wing Fortress, she goes completely apeshit. The end result: a bunch of enemy Flying types lying in pieces in nursery, followed by a Psychic attack so powerful that it could best be described as a psychic nuke. That is followed up with an even more obscenely powerful Fire (or Dragon) attack that completely obliterates Wing Fortress and a large piece of the surrounding forest. It is utterly terrifying but at the same time fuckingawesome.
In the Yu-Gi-Oh! GX Spirit Caller fan fiction, the protagonist Kaigen Kou is usually calm and slow to anger, but bullying or mistreating his friends releases a terribly ruthless and somewhat psychotic side. It's a bad idea to bully his friends, but sully the reputation of the woman he loves and you'll find yourself beaten half to death and inches away from being LIT ON FIRE.
Aziraphale's expression was like thunder. "Come to me, my child," he said, his tone as cutting as diamonds. "And I will show you a true wonder of a miracle!"
In Fill the Moon, when Demyx happens to witness Xigbar being slightly wounded by Captain Barbossa, that minor injury towards his lover is enough to get him to snap, summoning a hurricane and capsizing the Black Pearl, along with drowning around one hundred Heartless and keeping the entire pirate crew at bay with the power of his magic. Don't piss off Demmy-kins.
Another nice, cute, sweet messiah you don't piss off? The little girl called Senayax. Once you lay a finger on her family or her pet Heartless, you are in for an absolute world of hurt. Hollowmoon, the Eldritch Abomination that lives inside her, and Kingdom Hearts itself? They found that fact out the hard way.
Granted, Eija Satomi from Divine Blood isn't adverse to dealing lethal force when necessary, but she is still extremely friendly, patient, kind and generally feels remorse when she is forced to kill. However, after the ghost of her second, divinemother killed the first blood-related adult other than her own mother, apparently out of no reason than spite, Eija's response was violent enough to catch the attention of an ongoing psychic battle between the soul(s) of a Humanoid Abomination and several Gods and Demons. The Humanoid Abomination ran away while almost everybody else watched Eija seal the ghost by drowning it in psychic representations of blood.
Later,Kasumi kills a banquet hall full of rebel gods using what appears to be blood magic-based poison, given that it was triggered by stabbing her own hand, which caused them to sudddenly keel over one by one.
Uriel in Manchester Lost. One moment he is falling off the edge of heaven (which Raphael notes has no edges) and forgetting how to fly, as well as being a general Cloud Cuckoolander the next he is throwing the 3rd most powerful demon in hell through several buildings and the remains are sizzling and in bits. Then, after a long battle, he nearly succeeds in ripping Lucifer in half, and throws him all over the place.
The dwarven noble protagonist in Dragon Age The Crown Of Thorns expected Faren, the DC, to hate him when he revealed to him that he always knew Rica was Bhelen's concubine and had never said anything. Faren doesn't really end up hating him but, since he hates Bhelen's guts (among other, deeper reasons), he still delivers a No-Holds-Barred Beatdown to the prince, leaving him with a broken jaw and a couple of snapped ribs for good measure, plus a swollen eye. At least Faren ends up regretting it, after getting a What the Hell, Hero? speech from Kallian Tabirs, the city elf warden and his love interest, plus finding out just why the guy withheld that from him. He only really realizes just how badly he flipped out after remembering that the DN has a particular problem that prevents healing magic from working on him, so Wynne can't just snap her fingers and make the horrid pain go away.
In Reflections, Luna cares deeply for Celestia. She also won't let her sister run from her problems.
In Welcome To The Brothel, the protagonist takes time to write letters to his mother that showcase just how truly vulnerable and sensitive he really is. That doesn't stop him from being a highly effective warrior.
DC Nation - Dr. Light had unleashed a toxin that was killing all prepubescent children in North America as revenge for his Mind Rape at the hands of the JLA, just to make sure that the heroes' children suffered. He captured Sue Dibny to "finish what he started." A JLA strike team was sent after him. Light had them all on the ropes and was dangerously close to killing the Elongated Man when Sue broke out of her holding cell, and armed with Hal Jordan's ring, proceeded to open up a literal can of whoop-ass on him while cussing him out in English and Yiddish.
Nationverse actually loves this Trope. Hypertime elemental (and Cloud Cuckoo Lander) Shift is normally pretty laid back. When Shift's zombie grandfather almost killed him (during Shift and Miss Martian's first date date), Miss Martian morphed into her giant, scary white martian form and tore the zombie to little pieces. When the zombie grandpa got better moments later, Shift snapped and stranded him in a dead alternate Hypertime after beating him up at superspeed. He thought to himself (in his normal fragmented manner), after the blur of his own movements, "Tastes terrible. Zombies taste like old Slim Jims. Makes sense. Need mouthwash after this." The nice one bit off a chunk of his undead grandfather.
Another example is Fauna's "freak out" stage. Fauna is a sweet natured, Technical PacifistGranola Girl with abilities (she'll never call them powers) close to Beast Boy's. Push her to the point where her shapeshifting snaps her brain, and she will come at you with everything, including cobra venom.
The Doctor: Do that and you will end your days screaming in the deep places of my mind.
Big Bad: Y-you wouldn't do that! You can't!
The Doctor: Why not? You would.
Big Bad: B-but you're the Doctor!
The Doctor: Ah, yes, that argument. Heard that one before. And you know me so well, do you? You called me a monster not long ago. But now–ah, now you plead for the Doctor, the compassionate, merciful, pacifist Doctor.
Rise of the Galeforces gives us Sarah Squall, who at first glance looks and acts like a fun person to be around. But as various foes can attest to, she is in fact a Cute Bruiser who can toss friggin' dinosaurs around like ragdolls. And fly. Well, she ISStratogale after all...
The Western Animation section provides more details on the OTHER Super Family Team, but special mention goes to Violet in this case, who develops into a Genre SavvyCombat Pragmatist. Special note goes to her literalRoaring Rampage of Revenge in Episode 4, which ends when, after single-handedly wiping the floor with the last of the Big Bad's armada, she gets a yanked chain and a Torso With A View thanks to his final bullet... and her temper goes completely off the rails, causing her to order him slaughtered, horribly and on-screen, in front of the entire cast. Shy as she is, this is the kind of girl who won't take an insult lying down, and isn't at all afraid to kill if she has to.
Fluttershy in the Pony POV Series, as can be expected. Taken to logical conclusion when she snaps completely, getting corrupted in the same way as Princess Luna was and turning into the second Big Bad, Princess Gaia/Nightmare Whisper.
Later (or earlier, as it may be), during the Gaiden: Seven Dreams/Nightmares side story, Patch Took a Level in Badass, but still remains the goodhearted clown she's always been. However, her search for the fragments of the Rainbow Of Light leads her to the Pegasus Despot using the Blue Shard of Laughter to brainwash ponies and powering a city to become an Evil Overlord. He brainwashes her with it and makes her part of his brainwashed harem of mares, raping her repeatedly along with them. When she finally breaks free, she steals his fragment, shoots him in the leg, and leaves him to his Karmic Deathat the hooves of his now equally pissed off and unbrainwashed harem. She goes back to being herself afterwards, but it shows she's not someone to push too far.
Matthew Streika. Definitely one of the nicest, most decent people in a book full of things trying their very hardest to kill almost everything else, and is trying his best to make sure said dangerous things stay well away from his home town. All he really wants is to make sure innocents don't get involved, and protect his friends. He does not go looking for trouble. That said, do not mess with Matthew. Because when you manage to get him into a fighting situation, he is utterly ruthless and ferocious in a fight, not even minding his own pain until all hostile forces are either running with their tails between their legs or dead.
Uzumaki Naruto in White Rain is usually a happy, friendly, and goofy Hokage. When he stops being happy, friendly, and goofy, property values in Kusagakure drop in a hurry.
It's also considered to be a bad idea to piss off Jeri
Nia in Tengen Toppa Gurren Solvernia, in her first fight, she tries hard to talk the ganman who is striking the village to stop, seeing no sense in the destruction, but when he threathens to kill everyone in the village, Nia snaps and reduce him to scraps.
There's been no true hints to it in the story itself as of it, but Word Of God is that out of the main four in The Seven Names of Envy Angevin (Envy, Ed, Ling, Russell), Russell is actually the most dangerous, despite his general laziness, apathy and sarcasm.
Harry Potter And The Methods Of Rationality has an army. They're named The Sunshine Regiment, their badge is a smiley face, they make no secret that they're fighting for Good in the most sickeningly sweet way possible... and when the battle commences, they win. Their opponents are a child prodigy and a boy trained from birth as a death instrument.
Driad in Clash Of The Elements. The outer guardian of the Frozen Palace learned this the hard way when Driad regained his memories of his life as an outer guardian and briefly returned to that form to combat him.
Dr. Watson in Mortality (along with every other relevant Sherlockian work by the author). Ordinarily, he's the nicest guy you'll ever meet. But you mess withSherlock Holmes, and may the Lord have mercy on your soul, because John H. Watson will not. In one chapter, he's captured a criminal to interrogate, and he plays the Deadly Doctor trope disturbingly straight and delivers a pretty scary Badass Boast:
“If your master’s actions destroy him whom I regard as — as the best and wisest man I have ever known… make no mistake that I shall hunt down, to a man, everyone who played a part in his destruction.”
MLP:FiM fanfics tend to be full of those.
In My Little Denarians Rarity is a proper lady and sickened at the idea of violence. But when her friends are in mortal danger, she is the first of the group to start using human-made firearms. There is also Twilight Sparkle, who doesn't want to kill anyone but at her power remains extremely dangerous (At one point, when one of her friends is injured, she flips out with a banishment spell, sending several dozen cultists to the Moon... unaware that Earth's moon has no atmosphere). Last but not least Cheerlee, who goes Mama Bear on an enemy stupid enough to take one of her students hostage.
The adult fic Xenophilia gives us Lyra Heartstrings. She is calm, friendly and reasonable, with a lot of sense of humor and a bit of snark. She is also a grand master of the Still Way, a unicorn telekinesis-based martial art, with enough control to Neck Snap you from across the street, without moving a muscle.
This is how Princess Luna is depicted in Progress. She is a kind-hearted alicorn, if socially awkward, but not many of the characters who get on her bad side remember the alicorn part until it's too late. Especially if her glasses are broken, as Angel has learnt.
The Immortal Game: Twilight Sparkle, having been trapped inside her mind and forced to watch as her Superpowered Evil Side, Nihilus, tried to kill her friends, all while mocking her about it, finally manages to retake control during a critical moment of the Climax Boss fight between Nihilus and her friends. Nihilus begs Twilight for mercy, but Twilight coldly refuses and mocks Nihilus, even as the Elements of Harmony are used to destroy her.
Which is nothing compared to what Twilight does to General Esteem, the stallion who infected her with the Artifact of Doom that created Nihilus in the first place. After giving him a "The Reason You Suck" Speech and forcing his mouth shut to prevent any pleas for mercy, she proceeds to kill him in a brutally painful manner.
Spike: WHY WOULD YOU LEAVE ME WITH SOMEONE LIKE HER? Sonic: WHY DO YOU THINK I SAID DON'T MAKE HER ANGRY? WHAT, YOU THINK I WAS BEING CUTE?
Uplifted, a Mass Effect fic set in the middle of World War 2 features a young Wehrmacht NCO named Heinrich Fuhrmann. Heinrich is a soft spoken, gentle giant, who frequently stammers, refuses to shoot wounded soldiers or the men trying to save them and frequently finds himself duped by his superior officer, Joachim Hoch. However, whatever you do, don't try to shoot him. You might find yourself getting a bayonet in your chest or hit by enough molotov cocktails and machine gun fire to kill a small army.
Sleepwalker is usually pretty level-headed. However, if you threaten his Heroic Host Rick Sheridan or Rick's closest friends and family, you will suffer.
Hinata in A Growing Affection is as sweet and kind as her canon self, though she does slowly become more assertive. She kills the immortal Madara Uchiha with a weaponized black hole Rasengan.
Hobbes in Calvin and Hobbes: The Series normally flees at any sign of danger and is generally kind of a drag. He also doesn't like Sherman, a snooty hamster, and the feeling is mutual. When Sherman pushes him too far in "Remember", he actually attempts to kill him, and likely would have if it weren't for Calvin's Time Pauser.
Jago from Ah!Archfall!. Nice guy. But at the bare minimum of his power, he flattened three bikers WHILE DRUNK. And if he uses Jupiter...
