The titular Guardians are pretty nice, but if you threaten the children under their care, like Pitch and his nightmares, they pull out all the stops, like North's Blitzen Blades and Bunny's Egg-sploders.
Sandman is arguably the nicest one of all of them, but deliberately giving kids traumatic nightmares will have him beat the crap out of you with no hesitation and a fraction of the theatrics or even effort the other guardians use.
Friar Tuck. A jolly and compassionate fat friar, but when the Sheriff takes the last farthing from the poor box at the church for Prince John, and says that Tuck is likely to preach himself into a hangman's noose…
Friar Tuck: GET OUTTA MY CHURCH! (belly bumps him out the door, and begins beating him with a pole) YOU WANT TAXES?! I'LL GIVE YOU TAXES!
At first glance, Bob Parr is your everyday sitcom family man - kind, helpful, loves his kids, sucks at his job. Threaten his wife and kids, however, and he will ANNIHILATE YOU, as an unfortunate Syndrome finds out...
EVERY SINGLE ONE of the Parr family members, and yes, that includes thebaby as well.Action Mom Helen takes out four of Syndrome's mooks despite being caught in two closing doors, aptly namedViolet later manages to beat another one down with a stick, and Dash runs circles around a gaggle of them due to Deadly Dodging skills coupled with his ability to run on water. And let's not forget that Punctuated Pounding in his sister's defense just a few scenes later.
It's pointed out in the film that the Iron Giant was actually intended to be a ludicrously powerful defensive device. It's your best friend so long as you don't point something identifiable as a weapon at it or its friends. Once you do that though, the only thing that will save your puny, pitiful life is The Power of Friendship
The Lion King: Pumbaa, Simba, Nala, and Rafiki all show shades of this, especially during the Battle of Pride Rock. Screaming Mandrill Fu, anyone?
Film - Live Action
In 90% of his movies, Jackie Chan portrays a genuinely well-meaning and kind protagonist who avoids violence as much as possible, and never kills his enemies. But when pushed to fights, he can be a fierce warrior, and give his enemies an incredible beatdown, especially when angered. This is lampshaded in Mr. Nice Guy's trailer.
Forrest Gump is the most even-tempered individual you could ever hope to meet, but he has a Berserk Button when it comes to anyone hurting Jenny. He is also incredibly strong after being a football star and war hero.
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: 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!
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.
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.
In a more general sense, the whole Mon Calamari race. As exemplified numerous times in the movies and Expanded Universe, the Mon Cals are peaceful by nature and would like nothing more than to live in aquatic bliss in their homeworld's seas. But push them enough, or become evil enough that they start believing smacking you down to be the only morally just decision, and they turn into capable soldiers, master tacticians and the best warship builders in the universe. Then they build up their military and wipe the floor with you - which, needless to say, has elicited massive amounts of respect from the other races.
Mon Mothma: "Make no mistakes, the Mon Calamari saved the galaxy".
Rey from The Force Awakens. A naïve scavenger girl who's friendly towards Han and Finn, and who beats Kylo Ren during their first duel. And without any Force training.
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. In 'Let The Monster Rise', when he realises that Rotti has stolen Shilo from him, he completely snaps. It ain't pretty.
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.
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 City. 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...
The plot twist in Harry Potter and the Philosopher's Stone with Quirrell. He is quiet and nervous throughout the entire film but then it is discovered that it was all a facade: HE is in fact the bad guy and not Snape, who the main characters suspect all through the film, and also he has Voldemort ON THE BACK OF HIS HEAD!!!
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. In Revenge, he fights Megatron, Starscream and Grindor, all at the same time. To a standstill. And kills Grindor. And comes close to killing Starscream before getting backstabbed by Megatron.
"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. Also at the end of DOTM when Megatron attempts to broker a truce between the two factions:
Megatron: Besides, who would you be without me, Prime?
Optimus Prime: Time to find out. (Optimus grabs his axe, buries it in Megatron's head and then uses it to yank it off his body.)
Another great example from the Transformers film series is Dark of the Moon's Dutch. Seems meek and mild, until you put him in danger while he's trying to save the world. "That was the old me," indeed.
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 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. Later on, he saves his dad's life by blowing a bad guy's chest out with a shotgun.
Straw Dogs 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.
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, National Lampoon's 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.
In Rainer Werner 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.)
Maria from West Side Story is the sweet 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 Tony killed by Chino at the end of the movie, she completely blows up in front of everyone.
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 Charlie Walker. He is just a normal teenager and a Randy clone, which he basically acts like The Meta Guy. Then he stabs KirbyReed in the stomach, twice. And then reveals himself to be one of the Ghostfaces. Damn.
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:
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] 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!
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.
Edward Scissorhands is a shy, artistic soul who lets everyone push him around. He seems to have no idea how to fight... until someone he loves is threatened. Then he'll kill the offender with a single arm thrust.
In one of the final scenes of the Israeli/German film Walk on Water, soft-spoken, warmhearted Axel kills his Nazi Grandpa, responsible for the murder of thousands of Jews, after discovering that his parents have helped the war criminal to escape justice for decades.
Played with in Witness, where a group of teenagers harass the Amish that John Book is staying with, not realizing Book is actually a cop in hiding, and has no qualms giving the main bully a well-deserved ass-whooping, which he promptly does.
X-Men: First Class: Magneto is on the receiving end of Beast's homicidal fury when he tries to compliment the latter's blue furry form (which Hank misinterprets as an insult), and McCoy strangles Erik in response. Only Xavier is able to talk Beast down. Jekyll and Hyde indeed.
Nightcrawler doesn't want to fight at the East Berlin cage match, but when he's forced to, he wipes the floor with Angel (who is a Blood Knight and a multiple victor). Kurt then apologizes profusely.
Jean is very fearful about the "fire" growing within her and the harm it can cause, but when Charles is on the threshold of death's door because of Apocalypse's mental assault, she releases it in all of its glory to rescue her father figure. The Nigh Invulnerable En Sabah Nur is reduced to ashes by the Phoenix's wrath.
Professor X is an All-Loving Hero who is extremely dangerous because of (and not in spite of) his magnanimity. Even when his telepathy is negated by Apocalypse's Psychic Block Defense, Charles can still use The Power of Love as a weapon and he defeats the self-proclaimed god with it. Ironically, Xavier is a conditional pacifist who has a knack for inciting others to engage in violence on his behalf. The foremost example of this is the Phoenix, who is his defender in the Alternate Timeline instead of his doom because he avoids the blunder of his original timeline's self. Instead of being motivated by fear and forcibly caging Jean's "dark power" (which brought out the worst in her), Charles is motivated by love and helps her to cope with it emotionally (which brings out the best in her). The Phoenix is the most powerful entity in the movie-verse to date, and now that Jean has gained mastery over it, by extension, it's also under Professor X's command. In the first film, Magneto belittled Xavier for being weak, but in Apocalypse's denouement, Erik recognizes his friend's strength after his love for Charles redeems him during the Final Battle ("You can convince me to do anything"). En Sabah Nur makes a fatal error in underestimating his captive because the former only measures power through brute force, and Xavier is so much more than his mutation; the latter proves to be the superior and the more lethal leader because of his empathy.
Django Unchained: Dr. King Schultz is possibly the nicest protagonist from a Tarantino movie. He's also a deadly bounty hunter who you should never draw your guns on, as the Speck brothers found out the hard way, and whose patience should never be crossed as Calvin Candie found out.
Horrible Bosses: Sometimes Dale, normally a very mild-mannered and sweet-natured person, snaps.
Harry Hart is one of the nicest characters in Kingsman: The Secret Service, when he and an entire church congregation are affected by Valentine's device which turns off their inhibitions and increases their rage, Harry is able to slaughter the vast majority of them quite easily and horrifyingly.
Superman in Man of Steel, as is consistent with any potrayal of the character. Even after he's seen rescuing a busload of his friends as a schoolkid, by hauling the bus out of water, he continues to be bullied, but demonstrates remarkable restraint for a boy his age to forego any retaliation, and as an adult, is unfailingly polite and cooperative, even to the military demanding his surrender (to which he obliges). At one point, he saves a soldier who'd been firing at him and sincerely asks if he's alright, much to the soldier's shock. But when Zod makes the mistake of threatening his mother, Clark literally flies into a rage, spearing Zod through two silos, breaking his helmet and subjecting him to a very painful Sensory Overload, all while coldlytelling him exactly what's happening to him.