Beware the Nice Ones/Film

"Shaun: What you doing, you stupid moron? Ed: Fuck off! Shaun: You fuck off! Fuck fucking off! I've spent my whole life sticking my neck out for you, and all you ever do is fuck things up! Fuck things up and make me look stupid! Well, I'm not going to put up with it anymore, okay?! Not today!"
 * The title character from The Iron Giant is a Gentle Giant that lost his memories when he crashlanded on earth and befriends a local boy.
 * Shaun in Shaun of the Dead, whilst not exactly sweet and gentle, is almost supernaturally willing to put up with the slobby, selfish and lazy behaviour of his best friend Ed, and will defend Ed to anyone who criticises Ed for these qualities. Then, Shaun has an epiphany, the Dead rise and start to claim the Earth, and Ed makes the mistake of pushing Shaun's tolerance of his self-centred and increasingly reckless behaviour a little too far when he takes a trivial phone call on his mobile and puts everyone at risk:
 * Shaun in Shaun of the Dead, whilst not exactly sweet and gentle, is almost supernaturally willing to put up with the slobby, selfish and lazy behaviour of his best friend Ed, and will defend Ed to anyone who criticises Ed for these qualities. Then, Shaun has an epiphany, the Dead rise and start to claim the Earth, and Ed makes the mistake of pushing Shaun's tolerance of his self-centred and increasingly reckless behaviour a little too far when he takes a trivial phone call on his mobile and puts everyone at risk:

"Silent Bob: THE SIGN! ON THE BACK OF THE CAR! SAID "CRITTERS! OF HOL-LY-WOOD"!!! YOU DUMB FUCK!!!"
 * In a similar vein, Silent Bob, of the various movies made by Kevin Smith, finally reaches his limit for Jay's abuse and idiocy and yells at Jay. Though the explosion is short-lived, it is the only time Silent Bob raises his voice.

"Murray Futterman: What happened to him? Billy: I don't know. I guess they pushed him too far."
 * This example is also one of the few times that we see Jay -- who never stops crudely mouthing off and throwing his weight around -- shocked into meek compliance.
 * Similarly, whenever there is fighting to be done in Dogma, Silent Bob is in the thick of it. He knocks out the Golgothan with his trusty deodorant spray, and throws both Bartleby and Loki from the commuter train. You doubt? Just witness the look Silent Bob gives Bartleby and Loki when they knock out Jay, right before he gets up to tackle one of them.
 * "No ticket." Hee...
 * Yoda, when facing a Sith Lord in Star Wars: Attack of the Clones. In Revenge of the Sith, he gets even better, decapitating two troopers with a single blow!
 * Speaking of Star Wars, remember that sweet kid Anakin in Episode I? Whatever happened to him?
 * And in the original trilogy, Luke is trying to fight Vader calmly and peacefully. Until Vader threatens to corrupt Leia. Luke promptly bellows, charges, beats Vader down and lops off his hand.
 * Hey, don't you forget about R2! In Revenge of the Sith, he's captured by two assault droids who call him a "stupid little astrodroid". Cue R2 spraying them with oil and firing his jets. Result: two extra crispy and very much dead assault droids.
 * Do NOT call Pumbaa a pig or you will get your ass handed to you in so many ways.
 * Also, Simba and Nala, and Rafiki, they prefer not to fight, but boy, when they do...
 * In Sky High, Leila refuses to use her plant-control powers for violence, even when being taunted by the self-replicating cheerleader Penny - until Penny slaps her.
 * Repo! The Genetic Opera has Nathan Wallace, a sweet, loving, gentle, somewhat campy man who dotes on his daughter, Shilo. He also happens to be a Repo Man..
 * Gremlins 2 The New Batch. Gizmo, tired of the Gremlins abusing his gentle nature, fashions a flaming arrow out of office materials and burns Spider Mohawk alive.

"Wyatt Earp: All right, Clanton... you called down the thunder, well now you've got it! [points to badge] You see that? It says United States Marshal! Ike Clanton: [terrified, pleading] Wyatt, please, I... Wyatt Earp: [points to Stilwell, dead] Take a good look at him, Ike... 'cause that's how you're gonna end up! [he kicks Ike] The Cowboys are finished, you understand? I see a red sash, I kill the man wearin' it! [Ike flees for his life] So run, you cur... RUN! Tell all the other curs the law's comin'! You tell 'em I'M coming... and hell's coming with me, you hear?... Hell's coming with me!"
 * At the beginning of Death Wish, Paul Kersey is a kind, patient man who loves his wife and daughter. A conscientious objector, he served in the Korean War as a battlefield medic, and currently makes his living as an architect in New York. One day, his home is broken into by three men who rape his daughter and kill his wife. Kersey goes to town on the local criminal scum, killing eleven men before being told by the police to leave town. But it seems there are still thieves and murderers in Chicago...
 * Ditto the remake The Brave One, where the lead is Jodie Foster, and it's her fiance who gets killed.
 * John Candy had moments like this in his movies sometimes.
 * Pretty much the entire plot of Carrie.
 * Optimus Prime in Transformers: Revenge of the Fallen. Sure, he's Optimus Prime, who willingly takes orders from humans, and would like nothing more than to find a diplomatic solution to the war that wouldn't mean killing any more of his kind, but let us not forget that he's supposed to be the single greatest Autobot warrior of all time. Don't believe me? In Revenge, he
 * "GIVE ME YOUR FACE!" is about the most disturbing yet awesome thing you can hear Optimus Prime saying. Also worth mentioning, Prime's threatening challenge: "I'LL TAKE YOU ALL ON!"
 * DOTM: "We will kill them all." That is the most disturbing yet awesome thing you can hear Optimus Prime say. Epic amounts of slagging soon follow.
 * The 1960 Japanese film Yoshiwara: The Pleasure Quarter, directed by Tomu Uchida, can best be described as, "Picture The Blue Angel if it had ended with Emil Jannings taking a samurai sword and going medieval on Marlene Dietrich's ass."
 * In the first Transporter, Frank Martin completes a delivery, then is asked to carry another package .. which blows up his car at a chance moment he's not driving it. Unstoppable Rage ensues. Also Curb Stomp Battle.
 * Jim from 28 Days Later is a fairly gentle-natured guy right up until the last part of the movie when he goes batshit crazy and starts sticking his thumbs in people's eyes. In fairness, he has been through rather a lot
 * In A History of Violence, Tom Stall looks like he might count, but his dangerous persona is dangerous all the time. For a straight example, his son. Nice, nerdy, would rather make fun of himself than get in a fight. Gets pushed too far and puts the quarterback school jock in the hospital, and assault and battery charges are mentioned.
 * Somehow we managed to get this far without mentioning Straw Dogs, which could be one of those over-the-top Public Service Announcements on Bewaring The Nice Ones. Dustin Hoffman is a mild-mannered American tourist Fish Out of Water in a close-knit rural English village. He puts up with all kinds of crap from them with barely an angry word. Then they invade his home... and break his glasses.
 * This is the way the movie Mongol describes Temujin: for most of the movie he is a sweet kid and latter a nice fellow who just wants to live with his wife, but after being betrayed by father's warriors, having his wife kidnapped and raped and being reduced to slavery twice, he snaps and turns into a full fledged Magnificent Bastard, and manage to conquer/submit/destroy everything and everyone who treated him unfairly.
 * Many of the characters of Crocodile Dundee fit well here, especially Mick, Sue and Donk.
 * That poor abused housewife? Maybe you should stop pushing her once she's hit 50 feet.
 * Nishi in Akira Kurosawa's The Bad Sleep Well. So quiet, so polite, such a good secretary! But the film is influenced by Hamlet...
 * Clark Griswold, the patriarch and protagonist of National Lampoon's Vacation films, fits this trope very well. Though he is generally laid back and optimistic and just wants his family to have a good time even when they're indifferent to his efforts, Clark has been known to snap and go into into grossly profane tirades when he is sufficiently angered or pushed beyond the limit of his patience, causing his wife to remark "when you get mad you get weird." In the first film when his family wants to give up on the trip to the amusement park following a series of misshaps, he lashes out at them and, using a number of profane expletives, he declares that their trip is no longer a vacation but "a quest for fun." Then in the third film, Christmas Vacation, after his various efforts to assure a proper Christmas get together go awry, he has another violent meltdown in which he profanely denounces his boss.
 * Harold of the Harold and Kumar movies. You do not want to be on the other side of his Tranquil Fury.
 * Fresh: A 12-year old boy in a gangster-ruled ghetto, getting speed chess lessons from Samuel L. Jackson. Well, he's dead mea... wait, what?
 * Predators has a Predator clan putting on a planet a mercenary, a Israel Defense Force hitwoman, a Spetznaz commando, an African death squad trooper, a Mexican drug cartel enforcer, a death row criminal... and a doctor. Played by Topher Grace.
 * In Fassbinder's Berlin Alexanderplatz Franz Biberkopf is a once-feared, now-passive ex-con, just released after serving a sentence for the murder of his girlfriend. He endures insult on top of injury throughout the 12-hour film. Only in the epilogue, when it is revealed to him that much of his torment was intentional, does he crack and declare (while grinning and laughing maniacally) that he'll make his tormentor sorry. (Biberkopf is supposed to be a metaphor for the German people in the 1920s.)
 * Rob Roy MacGregor just wants to farm his land, play with his children, and make love to his beautiful wife. When some jerkass Englishman burns his house, rapes his wife, and generally destroys everything he has built up. . . .Excuse me, I'll be over in the next county.
 * Except that it almost became his undoing when he challenged Archibald Cunningham, the aforementioned jerkass Englishman, an expert swordsman and ruthless killer, to a duel. He quickly found himself overmatched, and it was only by pure luck and the unlikely success of grabbing the blade of the villain's sword at a crucial moment that saved him.
 * Maria from West Side Story is the sweet, somewhat bland, love interest to the protagonist, Tony. She doesn't seem to convey too much emotion at first (except when she finds out her brother is killed), but once she sees ) at the end of the movie, she completely blows up in front of everyone with an Anvilicious fury that you wouldn't believe.
 * From Undercover Brother Lance is probably the nicest and most friendly member of Undercover Brother's crew, but whatever you do don't under any circumstances ever call him a sissy, the result is not pretty. What makes it funny or rather disturbing, its Neil Patrick Harris that does the results. Says a lot what Dr. Horrible could do...
 * The Scream series seems to employ this with their killer(s). They are nice and friendly, able to help with whatever. But they harbor a lust for killing and target and kill those closest to Sidney Prescott or those closest to her or just people in the town she's in. Notable case is . is just normal teenager and  Damn.
 * Mothra is the only truly benevolent Kaiju in the Godzilla films preferring to live in peace on her island home with her fairy companions and her worshippers. That is, so long as you don't harm and/or kidnap said worshippers/airy companions. And, may Ghidorah have mercy on your soul if you dare harm her children.
 * Me Myself and Irene. The introvert, kind Charlie has put up with so much crap under his life that he finally simply snaps...and adapts a different personality.
 * In Tombstone, law-abiding Wyatt just wants to live a peaceful life with his brothers, and share a home with his lovely wife. He even turns the other cheek when The Cowboys start harassing his town. But when the "Cowboys" finally push him too far, he goes into this chilling tirade:


 * Taken: Brian Mills is a nice guy who retired from a high-stress, constantly on call government job to re-establish a relationship with his adored daughter. We see him shopping for her birthday present, barbecuing with some friends. He does use non-lethal force to bodyguard a pop star, but ends up comforting her like the father he is (and could be, to her). Then some Eurotrash scum kidnap his daughter, and though he offers them a chance to give her up and walk away, they don't. In the mayhem that follows, Mills doesn't flinch from shooting his FRIENDS when necessary. "Apologize to your wife for me," indeed.