Bernadette in The Big Bang Theory. Don't piss her off, she has access to weaponized smallpox. She also accidentally once crossed the common cold with the Ebola virus. Of course she wouldn't admit something like that because that would be very bad.
"Edith's Problem," the classic episode known not for its treatment of a typical American woman having trouble dealing with its symptoms, but Edith's irritability as she goes through the early stages. Most punctuated by Archie's well-meaning but ill-advised attempts to deal with his wife – oh, just "Stifle, stifle, STIFLE!"
Lionel Jefferson, the young black neighbor of the Bunkers who usually playfully tried to expose Archie's bigotry when Archie tried in his bull-in-a-china-shop way to mentor Lionel on the ways of the world. In "Lionel Steps Out," Lionel tells him exactly what he thinks of Archie's attitudes when he crosses the line and tries to stop him from dating his white niece.
The Brady Bunch: A real-life example of the nice-guy character being nowhere near as genial in real life, at least to creator Sherwood Schwartz and his son, Lloyd. Specifically speaking, Robert Reed, who played legendary model father Mike Brady. Mike Brady was a model father who was fair-minded and intensely loyal to his family, and out-of-character Reed was no different toward the child actors. While he was cordial with adult co-stars Florence Henderson and Ann B. Davis, Reed's working relationship was less than perfect. None of that compared to Reed's constant friction with the Schwartzes, a long line of directors and the writers, always over the scripts and the situations presented therein; the battles became a sense of he said-he said (Reed's claim the writers never had any realistic scripts and were more interested in inane slapstick, while the Schwartzes defended their work by saying their scripts were well-crafted and researched).
While Davis was the sweetest woman to her castmates (even Reed for the most part), there were well-documented instances of what life was like for people who got on her wrong side:
In Reed's case (documented in Barry Williams' memoir Growing Up Brady: I Was A Teenage Greg), it came during The Bradys era, after he had argued with the Schwartzes over a point in the script where Alice brings out a birthday cake with numerous lighted candles. Later in the scene, the script had Mike saying, "I don't know Alice, but we'll be in deep trouble ...," but Reed changed the line, switching the word "trouble" with "shit," causing Davis to snap back, "I don't know, Bob ... ''how deep IS your shit?!"
Davis reunited with the cast — minus Eve Plumb, who was replaced with a "fake Jan" — for The Brady Bunch Variety Hour, and was asked to do a series of skits alongside Rip Taylor, who played a neighbor named Mr. Merrill. Davis was not a fan of Taylor, to say the least; Susan Olsen's memoir on the series claimed that Davis considered Taylor's comedy skits to be vulgar and unfit for human audiences, and Davis refused to interact with Taylor in any way other than what was absolutely necessary for the show. To this day, Taylor has hard feelings for Davis and has maintained he was confused as to her attitude toward him. For the rest of her life, Davis refused to talk about her work with Taylor.
Breaking Bad: There's quite a couple of Affably Evil characters in the show that turn out to be capable of extreme violence. But probably no character exemplifies this trope like Todd from season five. He's a quiet and seemingly rather shy guy who is always nice and apologizes a lot, but the entire new drug opperation starts to turn into a disaster when he pulls a gun after stealing chemicals from a train to shot a 13 year old boy who saw them. He then arranges for the murder of Gus' nine henchmen in prison through his uncles gang and finally stops his uncle from executing Jesse because he first wants to torture him to learn what he told the DEA. He even seems nice when helping Jesse to get up and walk to his next torture session.
Buffy herself is very sweet and friendly but there is a reason vampires and demons are so afraid of her.
Willow full stop. After Riley gets her to help court Buffy she threatens to beat him to death with a shovel if he hurts her. This was after she wanted to ax murder Parker when he hurt Buffy. When Glory messes with her girlfriend she tries her very best to kill the attacker. This culminates into becoming season 6's Big Bad after Tara's murder. Warren, Tara's killer, gets flayed after she did some Cold-Blooded Torture on him. This transformation is foreshadowed a number of times: in the Season Three episode "Doppelgangland": the vampire version of Willow is sadistic; vampire personalities reflect the human's inner selves, with no soul.
In a Genre Savvy moment, ex-demon Anya says, "Well, those are the ones you have to watch out for the most. Responsible types. (...) Responsible people are always so concerned about being good all the time that when they finally get a taste of being bad, they can't get enough."
Tara may never have used a weapon before, but when a demon is trying to choke Willow, she will kill it. With a single strike. She later manages to shout down Anya.
Dawn can be scary if she wants, threatening Spike that if he does anything to her sister, he will wake up on fire one day. Spike is even scared. Poor Xander found this out the hard way when he was following Buffy's instructions to get Dawn out of town before the final throwdown with the First Evil. Dawn doesn't even wait to finish Buffy's note after she wakes up before reaching for her taser, zapping Xander, and turning the car around.
Quiet, stuttering, somewhat bumbling Giles efficiently beats the crap out of Ethan Rayne, then kicks him to get information. He goes after Angel with a flaming bat. He stabs the mayor in the chest with a sword. He delays Buffy so Principal Wood can beat Spike to death with his bare hands. He physically and professionally threatened Principal Snyder into readmitting Buffy back into Sunnydale High. He really proves it in season five. First, when a recalcitrant servant of the Big Bad refuses to talk, he tells the girls to get some rope; the camera follows them and we hear a noise; whip pan back to a now very talkative demon. He manhandled Spike while ordering him to get over his obsession with Buffy and move on. In the season 5 finale he suffocates innocent and long suffering Ben in order to kill Glory. After all, Giles used to go by the name Ripper.
Xander is always helpful and brave, but being non-superpowered, is not a formidable hand-to-hand fighter. Demons and vamps typically don't have to worry too much about him. But if you mess with someone he loves, you'll find yourself on the wrong end of his berserker rage. In "The Pack" he is released from his hyena curse and the first thing he sees is the crazed zoo-keeper attacking Willow. Xander attacks and strikes first, with Buffy finishing the job. In "Hush" Xander sees a bloody jowled Spike leaning over a napping Anya, his point of view suggesting that Spike had attacked her. Xander immediately launches a what-should-be-a-suicidal-attack-against-what-he-thinks-is-a-restored-vampire and starts pounding Spike. He sucker-punches an evil goddesswith a wrecking ball. After Angelus puts Willow into a coma at the end of Season 2, to Buffy: "If Willow dies, I will kill you."
Fred was usually sweet and gentle but on a few occasions she had her inner badass released.
In "Deep Down" she goes wacky on Connor with a taser when she discovers his complicity in Angel being sent to the bottom of the ocean.
Upon discovering in "Supersymmetry" that her physics professor sent her to Pylea, she plotted all sorts of nasty revenge, leading to the immortal line "You know what they say about payback? Well, I'm the bitch."
"Although I thought I might just shoot you in the throat instead. Now, if I pierce one of your carotid arteries, considering the temperature in here, 'cause I think somebody shot the thermostat, the blood loss is gonna be heavy. And there's a chance I'll puncture a vocal cord and you won't even be able to scream. But you'll want to when the blood loss to your brain results in a cerebral vascular event. That's a stroke. I wasn't trying to sound snooty."
In the hell dimension in which she was trapped for five years, the cave she hid out in had a handy nearby ravine that she used to dispose of bodies.
Lorne, the one member of the team who so rarely does any fighting.
He has this to say when Fred is lying on her deathbed: "If I hear one note, one quarter note, that tells me you had any involvement, these twonote Angel and Spike won't even have time to kill you. Oh and anything by Diane Warren will also result in your death."
Don't forget, if he ever hears you singing, Lorne will know all your secrets and usually some of your future. This makes his rare breaking speeches very effective.
Nothing really tops the moment when he shoots Lindsey in cold blood without so much as twitching. With a silenced pistol at that, all professional like. Did the guy deserve it? Probably. Was it out of character for Lorne to do it? Definitely. He makes it obvious that he thinks Angel has asked too much of him, but he still goes through with it as a final act of loyalty.
The Groosalugg is probably the nicest guy after Doyle, yet just as badass as Angel.
Community's Annie is adorable, yes, but don't you dare eat her beans. Or steal her pen.
And Shirley's a good Christian woman, but don't let that make you think she won't mess you up in paintball. And not just in paintball - "that thing about the jukebox was way too specific to be improvised."
And don't piss either of them off if they happen to be temporary campus security guards, as Jeff learned when their interrogation of him ended with Annie slamming his head against a table and Shirley threatening to cut him up with a pizza slicer.
Covert Affairs: Auggie is a good example of this trope. He's a sweetheart most of the time, but he has some unresolved anger issues that can occasionally make him violent. In the past three seasons he's...
...attacked (and almost killed) the man responsible for his blindness and the death of his friends,
...started a bar fight and smashed a beer bottle over a stranger's head,
...punched Arthur Campbell in the face,
...and got himself temporarily kicked out of Langley after he put Henry Wilcox in a choke hold.
Highway To Heaven: A third season, titled "That's Our Dad," could well have been inspired by Robert Reed's off-screen self. Said episode features Ned Beatty as Bill Cassidy, who plays a model father on-screen but is an insufferable jerk off-camera. Two kids who live in a foster home are ardent fans of "That's Our Dad" and want to be adopted (for real) by the actor, unaware that Cassidy's "adoption of two kids" at the end of each episode is a promotional trick. After the two foster kids learn the cruel truth about Cassidy, Jonathan helps set Cassidy straight and tells him he needs to live to his expectations.
Family Matters: A pre-Urkel episode titled "False Arrest" features Ron Glass as Buddy Goodrich, the star of an eponymously titled show that is an obvious parody ofDiff'rent Strokes (with Goodrich playing a wealthy black banker who adopts two poor white boys). On-screen, Goodrich plays the wise, genial father; off the set, Goodrich has a huge ego and is a jerk toward his castmates and others he works with. When Carl goes to the studio to procure tickets to the show, he nicely informs Goodrich that he's parked in a fire zone and needs to move his car, but Goodrich, believing he can park anywhere he pleases because he's Buddy Goodrich, refuses, prompting an argument; when Goodrich's ego gets the best of him, he takes a swing at Carl, prompting his arrest. Goodrich tries to use his charms to rally the other Winslows to his side, but the truth comes out and it becomes obvious he's just a jerk unworthy of their respect.
Game show hosts are sometimes accused of this, and there have been well-documented examples of several seemingly genial and/or jovial hosts being complete jerks off-screen. The most frequently cited example is Richard Dawson, with his well-publicized disputes with production staff and maniacal ego during his heyday on Family Feud. Bob Barker – particularly with his feud with the classic Barker's Beauties (and ex-production staff members who have sided with the Beauties in various lawsuits) and purportedly announcer Rod Roddy – has also been cited. In a more recent interview with retired producer Roger Dobkowitz, there have been revelations that Gene Rayburn (of Match Game fame) was a complete jerk to staff and others.
The Doctor is this trope breathing. The exact specifics vary a bit during the many incarnations of the character and series.
The Tenth Doctor is, for the most part, a cheerful, bubbly and fun-loving character — unless, of course, you happen to really piss him off, in which case he'll incinerate and drown your (overgrown spider) children ("The Runaway Bride") or make you immortal and subject you to a horrific eternal prison from which there is no escape ("The Family of Blood"), amongst others. "No second chances," indeed.
From "The Family of Blood"
Son of Mine: He never raised his voice. That was the worst thing - the fury of the Time Lord - and then we discovered why. Why this Doctor, who had fought with gods and demons, why he had run away from us and hidden. He was being kind. He wrapped my father in unbreakable chains forged in the heart of a dwarf star. He tricked my mother into the event horizon of a collapsing galaxy to be imprisoned there, forever. He still visits my sister, once a year, every year. I wonder if one day he might forgive her, but there she is. Can you see? He trapped her inside a mirror. Every mirror. If ever you look at your reflection and see something move behind you just for a second, that's her. That's always her. As for me, I was suspended in time and the Doctor put me to work standing over the fields of England, as their protector. We wanted to live forever. So the Doctor made sure we did.
Whenever one of his companions is threatened, you can bet that the Doctor will unleash some serious hurt.
Lampshaded in "Forest of the Dead" when the Doctor gives the villains an ultimatum to leave him alone or suffer. The villains, after realising just who the Doctor is, immediately back down and let him do whatever he wants.
The Tenth Doctor hits an all new level at the end of "The Waters of Mars". After getting tired of everyone dying around him, he decides to throw away the laws of time, declares himself the "Time Lord Victorious" in a speech that makes him in "The Runaway Bride" look sane, and saves a woman who was supposed to die. When confronted with his darker side, she kills herself in order to stop him.
In a number of episodes, the Doctor has at least attempted to simply announce who he is, expecting the villains to run away simply based on his reputation. It's even worked.
When the Weeping Angels mock the Eleventh Doctor and kill a perfectly innocent man, he gave them a Badass Boast, and when they don't listen, he proceeds to erase them from time.
One episode makes this part of its title: "A Good Man Goes to War." Turns out it's part of a saying in-universe that the villains have much reason to consider: "Demons run when a good man goes to war."
And in the actual content of that episode, the Doctor raises an army of hundreds of beings he's helped out in the past to take on an army who have kidnapped one of his companions to steal her baby to raise as a weapon against him. Though, the Doctor himself might argue that the last two words of this trope don't apply to him:
Good men don't need rules. Today is not the day to find out why I have so many.
In "Earthshock," the Fifth Doctor spends much of the story trying to talk the Cyberleader out of destroying the earth, but then when that fails... he rubs gold into his respirator, then shoots him. Twelve times. Just to be sure.
The Daleks, a race of aliens genetically engineered to be noting but pure hatred, completely devoid of mercy, pity, or remorse, serve as one of The Doctor's series long arch nemeses. There are billions of them and only one of him, and while they may not exactly fear him as such (since they're incapable of it), they certainly have a healthy respect for just how dangerous he is to them. They refer to The Doctor as "The Oncoming Storm" and "The Predator of the Daleks".
The War Doctor, the forgotten incarnation of the Doctor who fought in the Last Great Time War. When the Daleks notice he's arrived on the field of battle, they immediately begin freaking out.
Rose Tyler is a very nice, very normal 19-year-old Londoner. She also happens to be in love with the Doctor. When the Doctor is facing the Dalek fleet and tries to save Rose by sending her and the TARDIS back to 2005, she loses it and attempts to break into the TARDIS' central console. When she eventually does, she absorbs the entire Time Vortex to become the living embodiment of the TARDIS and travels to the future to destroy the Dalek fleet and resurrect Jack. Later, she even brags about it to some other Daleks.
Which companion would you expect to hold the Lord President of Gallifrey at gunpoint? Probably not Nyssa!
Amy Pond, who fits Violently Protective Girlfriend to perfection. Mess with Rory, and she will not hesitate to crush you. In "The Wedding of River Song," when the Silence are preparing to kill Rory mid Heroic Sacrifice (in an alternate timeline), taunting him that Amy will never come back, she remembers who he is to her and does indeed come back. With a fully loaded G-36 assault rifle. And opens fire, killing all of the Silence on the spot. Then, when Madame Kovarian begs Amy to save her from her malfunctioning eyedrive:
Amy: You took my baby from me, and hurt her, and now she's all grown up and she's fine, but I'll never see my baby again.
Kovarian: But you'll still save me though. Because he would, and you'd never do anything to disappoint your precious Doctor.
Amy: The Doctor is very precious to me, you're right. But do you know what else he is, Madame Kovarian? Not here. *reattaches eyedrive* River Song didn't get all of it from you, sweetie.
River Song plays with this trope. Every time we see her on camera she's very "Hello, Sweetie!" and playfully teasing the Doctor in an "I know something you don't know" kind of way. However, it's clear that at some point in her past, she's done something very bad, so much that when a Dalek (essentially pure evil in a metal canister) assumes, as a companion, she'll be compassionate and merciful, she simply states, "I'm River Song. Check your records again." It then proceeds to beg for mercy.
Rory Williams, who started out a nice fellow and a good nurse, and then got turned into an Auton with all of Rory's memories... and love for Amy. He guarded the Pandorica for two thousand years and then went hand-to-eyestalk with a Dalek that was threatening the Pandorica (and therefore Amy). The Dalek didn't come out of that one on top. And a note to any Cybermen reading this, DON'T make him repeat the question!
Rory kept wearing his Eye-drive long after it activated just so he could fight the Silence effectively and protect Amy, who doesn't even know who he is.
On top of that, who do you think the 'Good Man' in 'A Good Man Goes to War' actually referred to? Hint: The Doctor flat-out stated that it's not him!
Barbara Wright (later Barbara Chesterton). She's caring—and will commandeer a truck and mow down Dalek guards with it to get to her True Companions!
Ian Chesterton is an affable 1960s science teacher who functions as The Heart and spends most of his time looking after the others. He is also a terrifyingly gifted swordfighter and even capable of threatening the Doctor into submission when he has to.
Professor Chronotis in "Shada" is repeatedly described by the other characters as 'such a nice old man' - and he is, with a bit of old-man irascibility to balance it out. In reality he's a Time Lord outlaw possessing a completely unique psychic ability to forcibly rewrite other people's personalities with his own, and likely also a Charm Person. In the final moments of the story, the Doctor wonders to Romana that maybe someday people will look at him and describe him as 'such a nice old man'.
Captain Jack Harkness of Doctor Who and Torchwood. Normally cheerful, flirty and friendly. Hurt his team and he will do something horrible to you. Such as shoot you repeatedly with a pump action shot gun. In the knee caps.
Even though he's immortal, do NOT try to hurt Jack in any way. You don't want to find out what the usually soft-spoken, calm, and very loyal Ianto Jones will do to defend the man he loves.
In later series it becomes clear that Luke has inherited this from her. Don't try to isolate him and lock his friends up in horrible nightmares, and never trap his mum in a painting!
Stefan and Bonnie of The Vampire Diaries. They may be nice, caring and compassionate, but don't mess with them and the people they care about.
Elena. She is nice, but don't harm anyone she cares about.
Chuck seems harmless, friendly and Adorkable, but if you threaten his friends or family, expect him to destroy you. For examples, you may as well just look at the entire CMOA page for Chuck.
His sister Ellie usually wants to run from a fight, but if Chuck is in trouble, she quickly delivers the violence. She decisively ambushes Casey with a frying pan, mistakenly thinking he's evil. She clubs Daniel Shaw, one of the shows most dangerous villains, in the same style. She drives her car into another car to incapacitate Evil!Sarah.
Ellie's husband Devon is affable to the point of pacifism, blanching at the idea of holding a gun. But when a CIA guard needs to be dropped in order to save Sarah, Devon makes with the fist.
Carly Shay is very sassy at times, but she does have a long temper. To short it out is not a good idea. It was the focus of iThink They Kissed. Carly's always open to her best friends about her secrets, but when she learns of her best friends' secret that was kept from her, she gets "whipped up" about it. In such a state, NEVER tell her to calm down.
The oh, so nice Freddie Benson also seems to have his sadistic sides, especially towards Gibby (iWon't Cancel the Show) and Nevel.
Freddie: Because in 20 years, I guarantee you, I will be Carly's second husband.
Carly: What happened to my first husband?
Freddie: Nothing you can prove.
James May on Top Gear usually endures his co-presenters' antics with good humor, but when they really irritate him, he can be pretty mean. Case in point: In a challenge to buy cheap Alfa Romeos and enter them in a car show, May had carefully cleaned and polished his car and was using it to tow Richard Hammond's. After Hammond "accidentally" bumped into his car one too many times, May disconnected the tow rope without a word and left him stranded.
In the South American Adventure Special, James points out: 1) He is afraid of heights, 2) He doesn't like it when the others deliberately rear-end him, 3) they will be driving along a very narrow road adjacent to a very long drop, and 4) he is carrying a 3' long machete, which adds up to 10) if one of them rear-ends him on that road, he will castrate the offending party. Then Jeremy rear-ends him accidentally, the great oaf....
Also Richard Hammond, just once, during the race to the North Pole, although whether he'd actually have gone through with it...
Annie of Being Human. A ghost who is generally sweet and shy, who's never actually been in a fight, and comes across as being the most soft and kind of the threesome (the others are a vampire and a werewolf). But then she remembers her ex-fiancée killed her. At first her attempts to spook him meet with epic fail. Then she corners him, reveals what her flat mates are. And then she whispers something so horrifying to him that he breaks down in tears, runs screaming and BEGS the police to protect him. Bear in mind her ex had previously been completely unrattled by coming face to face with a ghost and was a monstrous Smug Snake with no redeemable traits... and she reduces him to a whimpering mess with a few words. DO NOT get on this girl's bad side.
Later, she finds out that the vampires have started to kill some people they were keeping as a herd; she shows up at their HQ in full on poltergeist mode, ripping doors out of their hinges and sending multiple vampires flying into walls hard enough to be knocked out. It was a Crowning Moment Of Awesome.
Werewolves in general seem to be this in Being Human, especially George and Tom. Both really nice, friendly guys, but George rips Herrick to shreds, and Tom has spent his entire life being taught how to hunt and kill vampires.
In the British detective series New Tricks, Jack Halford is a quiet, softly-spoken and wise old cop who acts as a mentor to the rest of the team... who possesses, if sufficiently roused, a fierce and at times even violent temper. Donottaunt him about his dead wife, either.
Superintendent Sandra Pullman is very nice, but that didn't stop her breaking the nose of a member of a biker gang who was holding a gun to her head, with her elbow, after talking him down.
It should be remembered that all three of the older detectives were respected and successful detectives during the time period of shows like Life On Mars and Ashes to Ashes. That's right, they are basically Gene Hunt with an extra thirty years of experience to mellow them out. But the Gene Hunt is still there somewhere. Particularly chilling is when Jack shows just how much he can intimidate a younger criminal, pointing out that while he might not be able to directly hurt the criminal, he could Let the criminal's name slip on the witness stand as helpful to the investigation. Which would lead to the criminal being killed or attacked in prison as a rat. What's even worse is that he says that he's done it before, and there is nothing to suggest that that's a bluff.
Supernatural: Sam Winchester - he's always got a kind word for someone shell-shocked from a brush with the supernatural, prefers to do research rather than pick locks and break faces, and will most certainly fuck you up if you even think about hurting his older brother. Just ask Gordon Walker (beheaded with barbed wire) or the Crossroads Demon (shot in the head) .
Castiel may seem amusingly out of touch much of the time, but you really don't want to make him angry. Hell, not even Castiel's True Companions are safe from this. At a perceived betrayal, Cas beats Dean to within an inch of his life. Cas going off the rails is NOT a pretty sight.
Dean: "Word of advice—don't piss off the nerd angels."
Get Smart: KAOS has created an evil robot and CONTROL's robot, Hymie, must stop him. Hymie decides he wants to be nice, and tries to make friends with the enemy robot. It does not work and the evil robot finally goes too far. At that point Hymie notes, "Hey! Nice is nice, but enough is enough!" and fights the robot and defeats it with the help of Maxwell Smart.
Space Cases: On the very second episode, the normally shy and self-effacing Radu gets infected with a virus, planted onto a teddy bear by an alien race as a form of biological warfare, and goes berserk. Before his illness-induced rampage is halted, the 'recreation room' is trashed, the ship's dotty android is in pieces, and Harlan almost gets Thrown Out the Airlock.
Dan, whose conversion as an Ultimate Insider in season 3 is in full bloom in season 4. It was either "adapt or die". Usually, something happens to remind him that he has a moral compass before he goes off the deep end.
River's brother, Simon, is generally meek, polite (if snarky) and occasionally clumsy guy who apparently wouldn't hurt a fly... mess with his sister though and you'll find that he not only throws a mean punch but can also get devilishly creative with his medical inventory.
His Future Badass self in "Five Years Gone" should have been a warning...
Adam himself, arguably. He comes across as a friendly guy, decent and caring (at least in modern times; in the past he's been a mercenary on at least four separate occasions) - he just happens to want to wipe out billions of people and choose who survives to live in his new perfect world, is all. Hiro's too much of a Cape to do that for pure revenge - this is a guy the world does not need to have turning up again.
The really cool thing about this scene is how he's taking blows from a guy with super strength to protect his son. When that guy crushes his son with a table, Sylar steals his ability, beats the living crap out of him, then FRICKIN EXPLODES. Don't kill his son.
Only character to come out of Volume 3 BETTER. Who'da thunk it?
From the graphic novels, Linda Tarvara. A friendly, personable young girl with a caring demeanour who is often polite to people. She's a dedicated and takes pride in her work. She also murders people by RIPPING OUT THEIR SOULS so she can FEED on their abilities. Her first human victim? A sweet, lovely old woman who she tricks, traps and murders, all while acting kind and considerate. This girl makes Sylar look like a candidate for sainthood (Season 1 Sylar mind you, the emotionless murder machine, not the "Morally Grey" character of Season 3)
Matt Parkman. Good natured, loving surrogate father to Molly and a man who desperately wants to be the best husband he can. But: Mess with his family or friends and expect trouble, as the fascist soldiers under the command of Emile Danko learned when Matt forced them to slaughter each other.
Or with Sylar. Locking him in a prison within his own mind, an empty world where years pass for every hour of real time, and then sealing his body behind a brick wall should do the trick. For two episodes, anyway. Mind you, Sylar borrowed Claire's Healing Factor, and used his newfound shapeshifting ability to move the part of the brain that can be attacked to disable it, so it's not like a bullet will bother him. That's the thing about some Moral Dissonance in Heroes: some of these villains are so powerful that you have to get creative to be sure they won't be back to relieve you of your gray matter someday. Notice that even these things almost never work!
Claire Bennet has shades of this. While she just wants a normal life, she's also quick to make use of her powers to protect people she loves... or destroy those who have hurt her. Take Brody Mitchum, who tried to rape her, and turned out to have raped someone else. She drove his car into a brick wall. While he was in it.
Let's not forget playing a major part in the bringing down of a plane, rescuing Eric Doyle from pursuing gun-toting agents, and bringing serious harm to Sylar on several occasions (including putting a pencil to the nastiest use since Rachel McAdams in Red Eye).
She is immortal, feels no pain, and is practically fearless. Not someone you want as an enemy.
Mohinder, anyone? Sure, he's become a bit of a Butt Monkey and is frequently handed the Idiot Ball. But let's not forget how he smiled and laughed and acted completely sweet and naive around Sylar... right before serving Sylar sedative-laced tea, strapping him to a chair, torturing him with a tuning fork, and then ramming a huge needle into Sylar's spine. And laughing about it. Damn, Mohinder.
In Ideal Steve is part of a small gang and is the buttmonkey. At one point he snaps cuts off Cartoon Head's ear (he's got a mouse face glued on his head so think more plastic, less blood) and knocks out Psycho Paul and becomes the leader of the gang for a few episodes. It turns out he just can't keep up the tough guy facade and tries to get everybody back to normal by apologising and asking for Paul to become leader again. Paul accepts but not before removing one of his eyes. Did I mention the show is a comedy.
President Jed Bartlet. Most of the time, a charming, amiable and folksy man with genuine affection for his staff (and who is greatly respected and admired by them in return), with a keen intelligence and encyclopedic knowledge of trivia. However, in the second episode, terrorists shot down a US military medical plane with the loss of life of all on board, including Bartlet's personal physician, for whom he had a great deal of affection for. And a different side of Bartlet emerged:
[chillingly calm] I am not frightened. I am going to blow them off the face of the Earth with the fury of God's own thunder.
And pretty much all of the next episode revolves around Bartlet's increasing desire for his military aides to devise a plan which will literally wipe them off the face of the planet, and his frustration when they tell him that this is neither practical nor politically desirable.
On a geo-political level, don't mess with people who are weaker than you or President Bartlet will remind you that most of the world is weaker than the United States Military.
Don't disrespect people in The White House. If you do, make sure you don't do it in front of Charlie, or he'll slam you into a wall and verbally take your head off.
All of the main cast of NCIS, being a team of Bunny Ears Lawyers, are capable of surprising any antagonist with their competence (especially those who surprise them first), but especially notable is Abby, the Perky GothLab Rat who, as a civilian scientist and all-around Genki Girl, seems like an easy target... and neatly puts her steel-toed boots to almost every villain who's made that unfortunate assumption.
"Always remember; I am one of the few people on Earth who can kill you and leave no forensic evidence."
Frank Black from Millennium is mild-mannered, law-abiding, honorable, and dearly loves his family. It's also heavily implied through the first season that the reason he is so gifted in understanding the minds of serial killers is that he has the traits of a killer himself ("I become the capability, I become the horror, what we know we can become in our heart of darkness") culminating in him brutally butchering the man who kidnapped his wife.
Spock in Star Trek: The Original Series. Generally a pacifist, but the very few times he loses his temper make it blatantly obvious why Vulcans keep such strong control over their emotions. And that's not even mentioning the times when something threatens Kirk and the pacifist Vulcan becomes rapidly unpacifist.
A key example would be Devil in the Dark where Spock went out of his way to try to protect the creature that was sabotaging the mine station and killing the miners, even attempting to defy Kirk and have it brought in unharmed. Once Kirk was facing the creature, though, even though it's not even moving, Spock immediately starts baying for its blood and has to be ordered not to attack it. Thankfully it turns out the creature was just trying to protect her eggs and it all works out for the best.
One discussing the whole "Who's the best Captain in Star Trek?" thing, many people dismiss Picard because they're not Genre Savvy enough to understand he's an example of this trope in action. It's understandable. You have rough-and-tumble guys like Sisko, a cowgirl in Janeway, and the uber-badass Kirk. Then you have this skinny bald Frenchman, constantly sipping Earl Grey, reciting Shakespeare, and all in all being a consummate gentleman and diplomat. It's easy to miss the fact that he's (deep breath) cursed out Klingons (in Klingon), killed Klingons bare-handed, survived Borg assimilation, survived torture, survived taking a large knife to the heart, told Romulans where to stick it (and backed it up), has single-handedly thwarted having his ship hijacked (several times), has told Starfleet officers several ranks above him to shove it (with not so much as a reprimand to show for it), has caused omnipotent aliens to bow to his gangsta, and has outfoxed at least one member of every known species in the Alpha Quadrant at least once. Beware the tea sipping Shakespearean gentleman, indeed.
And arguably, the fact that he concludes all these feats of badassery by sitting in his ready room, ordering up a cup of "Tea, Earl Grey hot!" And reading some Shakespeare makes him even MORE badass.
Which is why most "who's the best" is always between Kirk and Picard with no real clear winner.
Family Guy neatly summed it up in one episode when Neil, for a school presentation, brought his opinion on why Kirk is the superior captain, citing his "ruff-and-tumble style of command". His teacher dismissed the report as pointless, then added that Picard is the superior officer.
In fact, The Federation itself counts as this. Despite bordering on a Mary Sue Topia at times in the earlier series, from Star Trek: Deep Space Nine onwards (some would say as far back as The Undiscovered Country), the consequences of pissing them off become very clear.
There are some hints as early as TOS, mainly through General Order 24: Starfleet has a command for ordering the destruction of all life on a planet, and it's not a fully theoretical thing, Constitution-class starships are indicated to be capable of carrying through with that order.
General Order 7 is the quarantine order for Talos IV. One cannot say that there is, strictly speaking, a death sentence for visiting Talos IV, because there would be no trial. Any vessel that tries to go there will be stopped by any means necessary. Any vessel that reaches Talos IV will be destroyed with all hands without communication. This is to contain a purely memetic threat, with the potential to destroy any civilization it infects, at the recommendation of the civilization already infected.
Despite having the strength of ten men and the ability to calculate frankly incredible statistics in mere seconds, Star Trek: The Next Generation's Lt. Commander Data is generally a nice, good natured, pacifistic guy, and it takes a lot to get him riled due to the whole no emotions thing. Anyone who doubts his ability to totally kick ass when necessary, however, should really, really watch the episode "Descent I & II". And "Redemption". And "The Most Toys", for that matter. Seriously just... don't screw with him, okay?
Data: I assume your handprint will open the door, whether you are conscious or not.
Quark feels this way about humans, and by proxy the Federation itself. Sure, as a whole the human race is a pretty decent bunch, but that's only during peace time. Push them too hard and you'll have some of the nastiest hitters in the galaxy. Specifically, the Federation may be all about peaceful coexistence and exploration, but remember that they "explore" with heavily-armed explorer vessels that can stand toe-to-toe with the dedicated warships of their contemporaries, and have a super secret organization that will commit genocide on their behalf if they screw with them too hard (though just how canon it's existence is, is still up for debate). When they build actual warships, it's their Escort Vessels that can slug it out with the dedicated warships of their contemporaries.
Quark: Let me tell you something about Hew-mons, nephew. They're a wonderful, friendly people... as long as their bellies are full and their holosuites are working. But take away their creature comforts... deprive them of food, sleep, sonic showers... put their lives in jeopardy over an extended period of time... and those same friendly, intelligent, wonderful people will become as nasty and violent as the most bloodthirsty Klingon. You don't believe me? Look at those faces, look at their eyes...
Basically, one way to look at it could be to say that humans are basically, in the 'Trek century, a bunch of fairly genuine pacifists who would rather use words than weapons... but the only reason that philosophy works is because they're so armed up to the back teeth that they don't NEED to use anger and violence as a line of defense. Then when enemies like the Borg come along and terrify the life out of us... well, then you get movies like First Contact.
Essentially, speak softly and pilot spacefaring death.
And Voyager gave us perpetual newbie and resident Butt Monkey Ensign Harry Kim... who in "The Chute" managed to hand beatdowns to hardened prison inmates in defense of his Heterosexual Life Partner Tom Paris, before finally snapping and turning on Paris himself. He was on aggression-enhancing drugs, but still. There was also his Heroic BSOD at the end of "Timeless."
Voyager also gave us Kes. The nicest girl you'll ever meet, but don't get on her bad side. A body-surfing warlord who had lived for centuries found out the hard way that an angry Kes is not someone you want to mess with.
In one episode, there's a group of aliens that have invaded the ship un-beknown to the crew and are using them to experiment on for new advances in medical science. Janeway though is treated to headaches so painful it's like having needles boring into her brain, which there are for her. The aliens wanted to see the result of making the sweet and controlled captain, snap. The result - her taking control of the entire ship and driving it through a star to destroy the ship with the aliens on it, forcing them to leave. The crew survives though.
Captain Janeway: You wanted to see the result of making me angry? Well here it is.
The Middle Man. An old-fashioned, milk-drinking, seldom-swearing, well-dressed, earnest, polite, and naive gentleman who tortured a mob leader for info in front of his own bar, while drinking a tall, frosty glass of milk.
Battlestar Galactica: You won't believe who overthrew the Galactica's military. Felix Gaeta! Not only that, he ordered the murder of President Roslin!
Vir Cotto from Babylon 5 is usually a milquetoast, but as the series progresses he gets to demonstrate more and more that you don't need to have a heroic physique to be badass.
Lennier also shows tendencies of this, when the Badass Bookworm decides to put aside the "bookworm" part of the trope.
Delenn is the seemingly mild-mannered religious caste Minbari, whose mentor once said: "If you want to make a point, why not make it so that no-one misses it?". She took it to heart by breaking a would-be rapist's finger, beating a group of attackers into unconsciousness with a quarterstaff, obliterating an enemy fleet (including civilian passengers) that was meddling in Minbari internal affairs, goading her only rival to kill himself to save her life, and by (ultimately unsuccessfully) waging genocide against humanity after they pushed her just a little bit too far. It's no wonder Delenn ultimately ruled the galaxy as her personal fiefdom for the better part of a century.
While Rodney McKay of Stargate Atlantis isn't really nice per se, he's the most non violent character on any offworld team, and his wide blue woobie eyes make him victim fodder, not to mention the many times he's kidnapped. But he will often unleash a barrage of infinite The Reason You Suck speeches at the bad guy, or even his own team members when particularly stressed. That said, please don't give him Wraith enzyme. Ever.
And while Dr. Daniel Jackson really wants to get to know you and your culture, is a very sweet and gentle guy, and would prefer not to fight, if he has to, he will mess you up at least as badly as his career-military teammates because of his fierce determination to do the right thing. The man will NOT give up, even if he has to sacrifice himself to do it. And then he will somehow turn up alive again, because he's just that kind of person.
He also calmly threatened to kill Apophis if he didn't tell him where Sha're was.
Sha're is basically Daniel's Berserk Button for much of the series. The whole of the first 5 seasons is basically his equivalent of a Roaring Rampage of Revenge. He's so nice, he barely even raised his voice, he just embarked on the genocide of the species that took his wife from him.
We get a first glimpse of this in a first season episode, where he and Carter are standing over an incubator of Goa'uld children. He comments that the Goa'uld will grow up and enslave humans, and Carter responds with "Yes, but if we kill them now then we're no better than they are." Jackson, after pausing for a second, seems to accept it, starts to walk away... and then he spins and fires a submachine gun burst into the incubator, killing the Goa'ulds. That's for my wife, bitch.
For that matter, Samantha Carter is a good-natured, genial nerdy type, but she's also career military and can kick your ass in a second flat. RepliCarter, Sam's evil galaxy conquering replicator double, shows that we should consider ourselves fortunate to have Sam on our side. After all, she blows up suns.
Also from Stargate Atlantis, Teyla tends to be more calm than Sheppard and Ronon, and much more likely to grant mercy to bad guys. That said, don't hit any of her many Berserk Buttons: her fellow Athosians, her team members, and, most importantly, her son, Torren. She did kick Michael off the central tower of Atlantis, after all.
The eponymous Merlin. Adorable, loyal, and a bit of a Dojikko, but God help you if you ever try to kill anyone he cares about, because that is the last thing you will ever do. His anger is sudden and his powers are fierce and he will blast you off the face of the Earth. Keep in mind that this is the most powerful sorcerer the world has ever known. If he wants to tear mountains apart, he can do it. Please don't make him.
Merlin: "You should not have killed my friend." (pulls lightning from the sky and blasts Nimueh to ashes)
He also has the highest body count in the series, can pretty much stop time, and can kill you just by looking at you. And do it with absolutely no emotion. In-Universe, he's a Lethal Joke Character.
Similarly, Clark Kent from Smallville (at least before a nonsensical transformation at the end of Season Eight) is a shy, humble, unassuming young man, generally avoiding the limelight and talking his way out of battles; even when he does fight, he'd rather use defensive maneuvers to defeat his foes. But when anything bad happens to a member of his family, from his parents to his friends to Lana, all bets are off.
Perhaps best summarized in the pilot episode (and opening) of the 1970s The Incredible Hulk. "Mr. McGee, don't make me angry. You wouldn't like me when I'm angry." Remember that in this version, the Hulk essentially is Banner, but with tremendous strength, and a child's mentality.
In House, Dr. James Wilson seems deeply kind, caring, and eternally compassionate to his patients, and nearly as much so to his friends. And he is, until he gets pushed far enough to reveal that deep down, he can be just as much of a cold, snarky manipulative bastard as House - maybe even more so. He just chooses not to be because it seems healthier.
Zhaan of Farscape is by far the most compassionate and level-headed member of the cast, and usually exudes a Zen master level of calm. But it's revealed halfway through the first season that she was a very violent person in the past, and even after she attained inner peace, every once in a while she loses her temper, showing a cold fury that is much creepier than any of the others outbursts.
Zhaan: Is this the way you repay my help? How would you like your arm torn off? Hear me! I could rip you apart! Right now, Kahalan help me, I'd enjoy it.
Likewise, Pilot is a shy and gentle creature dedicated to serving the crew of Moya, regardless of how much they exploit or abuse him. However, on quite a few occasions, the crew or unwanted visitors have found out the hard way that pissing off the guy that controls the life support systems might just be a bad idea. Case in point: the terrorist he flushed out of an airlock while laughing maniacally.
While burying Moya's son, they are attacked by an insane Leviathan for carrying other life forms. Dargo is out in his small gunship, preparing to blow it away, but everyone is worried Pilot and Moya will be upset if they kill the attacking Leviathan. Then Moya informs Pilot that the insane leviathan starved her own Pilot to death, and Pilot has two words to say on the whole situation:
Leo of Charmed. He is a peaceful healer who often gives words of advice to the Charmed Ones. But people forget that he was a medic during World War II and if anyone threatens Piper or his sons, he will kill.
He has killed an Elder and even helped a magical group wipe out free will.
Will and Finn on Glee are the epitome of Nice Guys. The two spend the first half of season one in The Baby Trap by their women and when they find out...whoo boy. Will goes borderline psychotic upon realizing his wife's fake pregnancy and it almost appears like he is going to hurt her. Finn meanwhile finds out his girlfriend's baby is really his best friend's and the second he hears it runs after him, attacks him, and proceeds to beat the living daylights out of him, be held back by several football players, and manages to chuck a chair on his way out.
Not to mention Burt Hummel, a Good Parent and Nice Guy, who chases Dave Karofsky down in the school hallway, pins him up against the wall and is about an inch away from beating him senseless after he finds out that Karofsky threatened to kill Kurt. And this just five episodes after a serious heart attack and coma. If you know what's good for you, you do not threaten Burt's son.
And then, let's not mention what the reaction of our group of misfits were when Rachel was egged by her ex-boyfriend and his rival glee team. The moral? Don't mess with New Directions.
They have a very similar reaction when Blaine gets hit by a rock salt filled slushie by Sebastian, his Stalker with a Crush and put into the hospital. All of them look ready to go start a fist fight, and Mike even says that they want an eye for an eye. They're very protective of their own.
Also, don't mess with season 4's quirky antiquarian T.P. Aquinas, who has spent years plotting his revenge against the drug lord who murdered his wife and young son. He succeeds when he detonates the car that the drug lord is riding in.
Deadwood: Seth Bullock runs up against the edge of "nice" (mixing this trope with Hair-Trigger Temper) but in general he's an honorable, hard working, polite man who helps out troubled widows, acts as law keeper for Deadwood, and always arranges a proper funeral for people who've tried to kill him. But push him a little too far—mess with his friends, or call him out on his extramarital affair—and you'll find out why his temper is considered literally psychotic even by cold-blooded murderers, often coming close to killing the offending party with his bare hands.
M*A*S*H: In the episode "Period of Adjustment," nice guy B.J. has enough of the war, gets nasty-drunk, throws darts at Radar in effigy, smashes the still with a chair, and punches his best friend in the mouth.
More consistent, yet less extreme, is Father Mulcahy. As expected of a sincere man of God, he strives to be compassionate, patient, and forgiving. Sometimes though, the right thing has to be done whether you like it or not, and if he can't convince you with wise words, a fist in your mouth makes an acceptable substitute. He feels pretty bad about it afterwards, but regardless.
And lets not forget the fact that before he was a priest, he was a boxer.
Let's not forget about Hawkeye, normally a nice guy with a penchant for pulling pranks. However, if he finds out you are causing harm to children, he doesn't care if you are in the army as well — you are going to get it.
There's also Radar, a shy but kind guy with a love for animals. Hurt said animals, though, and there's not much that can save you.
Also, don't shoot the bugle out of his hands with a cannon, even if it was by accident. He'll still need someone to hold him back.
Juliet in LOST also qualifies. As seen in her flashbacks, she used to be an incredibly mousy and docile person, frequently letting people (especially her ex-husband) walk all over her but all of that changed when she got to the island and met Ben Linus, who extended her six-month stay there to eternity. Since then and especially after discovering Ben's many other lies to her, including his involvement in her lover's death she took several levels in badass. The biggest evidence of the damage done to her comes in Season 5, when Sawyer's throwaway look at Kate turns out to be the last straw and Juliet decides that the only response she can muster is to detonate a hydrogen bomb.
Ben himself isn't nice per-see, but he certainly is polite and well spoken, if a little power hungry. However, mess with the people he cares about or dare try to harm the island or his people and he will get... ANGRY (Keamy found this out when he killed his daughter)
Arguably, Kate is an even bigger example: Her sweet girl-next-door demeanor is genuine, but so are her more violent and manipulative aspects.
Charlie counts as well. He and Hurley tie for Friendliest Survivor, but if you do anything to hurt Claire, Charlie will cut you down in cold blood.
And while we're on the subject, Hurley himself. Generally treated as a big cuddly teddy bear at best or The Load at worst by the other castaways, he has shown himself capable of delivering a No-Holds-Barred Beatdown to Sawyer (while screaming every unkind nickname Sawyer ever threw at him) and mowing down a bunch of Dharma thugs with his Winnebago van.
Blair Sandburg in The Sentinel. Anthropology grad student, raised by a mother who could be described as "the last flower child not gone to seed" — but threaten (or even worse, hurt) Jim and you'll wish you had left matters well enough alone.
Doctor Parker, from Flander's Company, is a very nice fellow, always calm and collected, happy to help his company and others in general... Until his Super-Powered Evil Side takes control. Expect a lot of slaughters and massive quantity of blood.
Mason is pretty nice, too, as long as you don't hit his Berserk Button.
Eric Gotts from Wonderfalls was by far the nicest, least obnoxious and least ridiculous of the main characters, but when his cheating ex-wife turned up, he was downright vicious to her, including throwing a glass across the room, narrowly missing her head.
Most, if not all, Power Rangers fit into this trope, with a special emphasis on the female rangers, but the best example would have to be Kimberly. A tiny wisp of a creature who doesn't appear to weigh 100 pounds, she acts like a 80s Valley Girl more times than not, but when threatened she becomes one of the toughest rangers ever. Made even more obvious with her weapon, The Power Bow. Her arrows have been shown to be able to turn corners and navigate through trees, meaning she has what amounts to a portable rocket launcher at her disposal.
Anyone watching the first season quickly rediscovers a season-long running theme. Whenever Kimberly shows up looking angry, the monster dies. In one example, Kimberly arrives to find the other five Rangers incapacitated. So she single-handedly annihilates an entire army of putties by shooting them one-by-one with her bow, then takes out the monster seconds later.
In Hawaii Five-0 (the new one) Steve is usually the crazy one who breaks a lot of laws while interrogating suspects, whilst Danny chastises him. However, if you hurt or threaten anyone Danny loves, especially his daughter... you might start to wish you'd never been born.
Rommie in Andromeda. A nice polite and considerate young woman who just happens to be able to blow up a planet.
Betty White's character Rose of The Golden Girls fame, is the sweetest, most bubbly member of the foursome, if also the dimmest. But there is a very, very good reason why the others take great pains not to get on her bad side...
Kamen Rider Kuuga's title character, Yuusuke Godai, is normally a kind-hearted guy who wants nothing more than to protect people's smiles, and is especially good with kids. But as one sadistic Grongi learned the hard way in episode 35, it is not a good idea to push him too far. Said Grongi ended up on the receiving end of one of the most frightening Unstoppable Rages of the entire franchise, a savage No-Holds-Barred Beatdown that culminated with Godai finishing him off with a rage-fueled Rising Calamity Titan attack.
Then there's the Linto tribe, pacifistic and nature-loving ancestors of humanity. The Grongi tribe has been systematically murdering the Linto as a macabre game. One day the Linto decide enough is enough, create the Arcle, and select a champion to wield it against the Grongi. That champion is Riku, who becomes the first Kuuga. Alone, Riku proceeds to comprehensively hand the Grongi their asses, defeat their leader (who is the strongest Grongi), and seal them (and himself) away for the next two thousand years, all without access to any of the Rising forms Godai has.
Yuka Osada from Kamen Rider Faiz also fits into this trope at times. Normally shes quiet and polite but hurt her friends or be a bully and it won't be pretty.
Russell Glasser of The Atheist Experience is often seen by theist callers as the safest one to talk too. This is a misconception as his comments cut deeper and deeper as he slowly loses his patience with you.
EADA Ben Stone of Law & Order is a real Nice Guy, especially when compared to his successors, Jerk Ass Jack McCoy and borderline Smug Snake Mike Cutter. But if you managed to push his buttons, watch out. He will destroy you, all while remaining perfectly calm.
And in similar fashion, across the pond, we have DS Matt Devlin, who's also basically a very Nice Guy, and even manages to remain so while chasing and interrogating suspects. But push one of his buttons (child abuse, Alesha Phillips, Ronnie Brooks), and it'll take every ounce of his self-control to not wring your neck.
(as he and Ronnie arrest Dr. Merrick, Alesha's rapist)
Merrick(whines as Matt cuffs him): "You're hurting me!"
Matt: "Yeah, I know."
(the fact that he smiles as he says/does this just drives the point home)
Game of Thrones: Daenerys Targaeryn may seem like a young, helpless girl. But if you go after her husband or her child, she will burn you alive.
Spartacus: Blood and Sand has the generally quiet and meek Aurelia, wife of Varro. She seems to be the only delicate flower in a ludus filled with gladiators, guards, and soldiers. Then we learn that she castrated her rapist. In the final episode she she brutally stabs to death the teenager who callously ordered Varro's death. Even Sparty and Doctore (you know - the deadliest gladiator in the republic, and the man who trained him) are kind of squicked by her rage when they arrive on scene.
On BBC's Sherlock, Doctor John Watson is an unassuming, gentle soul, who lets Sherlock order him around on their adventures. It's so easy to forget that he has recently seen active service in Afghanistan, and shoots a serial killer dead, between two buildings, through two windows, with a handgun, in the first episode of the show. And shows absolutely no remorse about it.
Actually, of the two major bad-guy deaths over the first three Sherlock episodes, BOTH of them were killed by John Watson. The second being in "The Blind Banker", where he successfully offs a Chinese gangster trying to simultaneously kill both Sarah and Sherlock, by one movement of his leg. Which happens to be tied to a chair at the time. Do not mess with John Watson or anyone he happens to like.
It's also a pretty bad idea to insult Sherlock in front of John. Just ask the Chief Superintendent of Scotland Yard.
Paladin of Have Gun — Will Travel is generally a fairly friendly guy, and even gentle to nice people, but can be a terrible enemy if you cross him/hurt innocents.
Many many Super Sentai characters are perfectly friendly Nice Guys, with some Jerkasses eventually revealed as having a heart of gold along the way, but if they're pushed too far, massive ass-kicking will ensue. However, the one that claims this trope ultimately is Ryuuji Iwasaki/Blue Buster from Tokumei Sentai Go-Busters. He's a perfectly calm, genuine Nice Guy who serves as a Team Dad, but his weak point is when he overheated... in which he becomes utterly scary, has a much more brutal, sadistic fighting style, will not stop even if the Monster of the Week is begging for mercy, until the monster is dead, and does not differentiate between friends and foe. When Yoko/Yellow Buster found out the hard way and nearly got her head squished to the wall even after believing their bonds would save the day (which was proven wrong), she broke down crying in shock.
Once, when facing Escape, one of the Co-Dragons, alone, he deliberately drove himself into his berserk state to become strong enough to win. That makes him cooler and scarier at once: don't push him, because every tool he has is on the table, even basically becoming The Hulk in a Ranger suit (with the risk of death if he stays in that mode too long!) This makes Escape decide he's interesting and want to fight him again; she becomes his personal enemy just because she considers him the only one good enough to be worth fighting.
Lane Pryce is quite the calm fellow, but if you press his buttons too hard, like Pete did in season 5, he will challenge you to a fist fight and he'll beat you. Pete found that out the hard way.
The main cast of Leverage is more or less this all the time: if you're their client, they'll move heaven and earth to help you; if you're their mark ... And heaven help you if you make them mad.
Blue Bloods Police Commissioner Frank Reagen hardly ever raises his voice, and for the most part his days of tossing & cuffing perps are long behind him. He's still capable of dealing out .38 caliber death to anyone who threatens his family. An unrepentant serial rapist who attacks Erin learns this the brain-splattery way.
Marshall of How I Met Your Mother is a very sweet lovable lug of a guy. But when Ted gets punched out in a fight, he punches the other guy out, much to the surprise of everyone. Turns out he routinely used to have brutal no-holds-barred beatdowns with his brothers for the hell of it.
Marshall set up a website with a countdown to one of his slaps months in advance, just to make Barney suffer as much as possible. Then when the moment arrived he tied Barney to the slapping throne, prolonged it as long as he could, allowed every other person the opportunity to slap him instead, then declared that there would be no slap, untied him, then slapped him so hard he fell over.
Marshall: That's two!
On JAG, Bud and Harriet are the sweetest couple and good parents, but both go on the warpath if someone screws over their spouse.
CSI: Crime Scene Investigation: D.B.Russell. He's normally kind of laid back, rather quirky and has a weird sense of humor sometimes, but when former undersheriff [=McKeen=] got his granddaughter kidnapped, he got out his gun and was on the edge of taking the kidnappers out himself. Brass had to rein him in a couple of times during the search.
CSI NY: This has been said about Lindsay Messer, from some of her conversations with Danny. One was about how she'd make him look like a domestic abuser and another was about how she'd haunt him if she died before him. There's also her protectiveness of her family these days.
After being screwed over by Regina and her mother Cora one time too many, including the murder of both her parents and the maid who was a mother figure to her after the fact, Snow finally retaliates and curses Cora's heart and tricks Regina into putting it back into her mother's body, killing her.
In Modern Family, Phil often tries to be the peacemaker, even when Haley is arrested for underage drinking, resisting arrest, and assaulting a police officer. But when she calls herself the victim, he chews her out, making her realize how irresponsible she's been.
Or the time Alex and Haley lied to him about finishing their chores so they could go out, prompting him to jump on the hood of a moving car to stop them. Or the time the kids wouldn't stop fighting on their RV trip. Phil is normally a pretty laid-back guy, so when he does lose it everybody is immediately cowed into frightened obedience.
In House of Anubis, Fabian is often believed to be one of the nicest people in the house, as well as an AdorkableExtreme Doormat. But if you threaten or insult Nina in his presence, he will do everything from simply shout at you, to honestly trying to attack you physically.
Grimm: Monroe is a bookish, adorkable watchmaker / clock repairman with a passion for Old Europe, cooking, wine and the finer things in life... and a werewolf who could - and sometimes does - tear people apart. Nick is a genuinely good cop who will leave you alone if you haven't done anything illegal, no matter who or what you are... but he will smash your face with a warhammer if you threaten his friends or family. Juliette, a veterinarian and thus a friend of all living things, but when an ogre breaks into her home she doesn't hesitate to throw a pan of boiling water in his face, and when her friend is being abused by her husband she beats the cr@p out of him using common kitchen utensils. Indeed, it is hard to think of any of the regulars or semi-regulars who [I]don't[/I] fit this trope to a greater or lesser extent!
Nobuharu Udo/Kyoryu Blue of Zyuden Sentai Kyoryuger may be the show's Plucky Comic Relief but in battle he's probably the most vicious fighter on the team. Within the first couple of episodes we see him shoot Mooks to death while they're struggling on the ground and stabbing them in the crotch with the nasty metal spikes on his shield.
Daigo as well. Even the Big Bad, Chaos, notes it in one episode: "Don't underestimate him. When he has that look in his eye, he is a force to be reckoned with."
Cole in Tracker. His race is normally peaceful and nonviolent. He is friendly, caring and often a bit bumbling as he isn't used to being human. But harm Mel in any way, and you will pay for it. Killing isn't his nature, but he'll still beat you up for it. And in the case of the fugitives, he can make the extraction process slow and painful as he did with Rhee, the killer of his wife and child.
Roy Desoto from Emergency! could fall into this at times. He was normally pretty easy to work with and get along with, but he would not hesitate to call someone out if they started trying to take things further than their training and patients got endangered. The main case was a Combat Medic who thought his battlefield training made him better than Roy and John. Roy never actually yelled, but he got a deadly serious look in his eyes and gave the guy a serious talking to with a tone that made it clear that was one of his angriest moments of the series. Not that it did much good.
It's not advised to keep harping on the nice ones in Cutthroat Kitchen. Piss them off enough and they will hit you with all the sabotages in the game.