Beware the Nice Ones/Fan Works

"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!""
 * 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.
 * 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.
 * The portrayal of Fluttershy as seen so far in Friendship Is Witchcraft.
 * Latias from Latias' Journey. She's depicted mostly as a gentle, somewhat timid and fearful Pokemon. That changes big time in Chapter 5. It is utterly terrifying but at the same time fucking awesome.
 * And killing the rest of the Pokemon she was trying to save.
 * 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.
 * This Crossover fic between Good Omens and Supernatural. Aziraphale unleashes some truly righteous fury on some wannabe Satanists who have crossed the Moral Event Horizon to summon the wrong yellow-eyed demon.

"The Doctor: Do that and you will end your days screaming in the deep places of my mind.
 * In Fill the Moon, when Demyx happens to witness that minor injury towards his lover is enough to get him to snap, summoning an entire 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.
 * 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, divine mother 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, Yamato Nadeshiko 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 Cloudcuckoolander the next
 * Aziraphale in the same fic. After he, he gets so angry, that even Beelzebub Lucifer's second is frightened of him. Unfortunately he is distracted.
 * The dwarven noble protagonist in Dragon Age the Crown of Thorns expected Faren, the DC, to hate him when he revealed to him that Faren doesn't really end up hating him but, since he hates Bhelen's guts (among other, deeper reasons), he still  At least Faren ends up regretting it, after getting a What the Hell, Hero? speech from Kallian Tabirs, the city elf warden and  plus finding out just why the guy withheld that from him. He only really realizes just how badly he flipped out after remembering that
 * 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 Cloudcuckoolander) 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 Pacifist Granola 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.
 * In Dumbledores Army and The Year of Darkness, Dennis Creevey is one of the sweetest, nicest boys you could ask for... until the Death Eaters finally push him too far. "AVADA KEDAVRA YOU SON OF A BITCH!"
 * In The Man With No Name, the villain utterly fails to do this in regards to the Doctor.

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 IS Stratogale after all...
 * It's probably not a good idea to mess with the rest of her family, either. Sarah's dad can drown you with a cupful of water and sic sea creatures on you, while her mom is a Gravity Master who can suck you into a black hole in ten seconds flat. Her uncle is a true dunderhead, but he can still kick your ass with either lightning, wind, water, or ice, whatever comes to mind first. Oh, and Terrordactyl? Suffice it to say that his name pretty much speaks for itself.
 * And don't start on the OTHER major family, either. See the Western Animation section for more details on the Parrs themselves.
 * Fluttershy in the Pony POV Series, as can be expected.
 * The Heroes Dark Fic Unmade gives us this in an instance of intense Psychological Horror when Emo Nice Guy Peter Petrelli (who also just happens to be a Person of Mass Destruction)
 * In any Adventures Of Blinky Bill fanfics written by Canzet Yote, it is NOT a good idea to piss off Shifty Dingo or Nutsy.
 * Kyon doesn't go looking for trouble. He really doesn't. But then someone goes after his Nakama, and he alternately beats them to a bloody pulp or dangles them out a window. And now he has.
 * 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.
 * In Princess Celestia Hates Tea, Celestia shows exactly why she is a Physical Goddess when wild accusations of being an imposter result in her entire staff, the Elements of Harmony, and her own sister trying to exorcize her. And the whole fiasco was over Celestia lying about liking tea to spare her sister's feelings.