Subverted at least twice in Superman comics (perhaps to be expected, featuring as it does the ultimate Nice Guy):
An issue of Superman entitled "What's So Funny About Truth, Justice and the American Way?" sees Superman challenged by a Darker and Edgier superteam who aren't afraid to kill and maim their enemies, and deride Superman as a moral weakling who's past it and afraid to deal with issues "properly". Eventually, challenging Superman to a fight, they pound and pound and seemingly break him — but, in a completely unstoppable explosion of pure superhuman rage, he seemingly destroys and kills each one of them, and uses his X-ray vision to completely destroy the tumor in the head of his opponent that was giving him his powers. However, it's revealed that Superman was still holding back. Each member of the "dark" team is alive and well (if somewhat battered) and the leader's powers still remain, and he has merely given them "a psychic concussion" — because, as he explains to the leader, he wanted to give them a sense of what it felt to be powerless under such unstoppable brutality (namely, the feeling their victims had and a sense of what it would be like if he was actually like that). It wouldn't be pretty.
A sequel to this story sees Manchester Black, the leader of the Elite, in an attempt to yet again break Superman, apparently killing Lois Lane. The issue follows an enraged Superman beating Black to a pulp before killing him. However, in yet another subversion, it's revealed that this was just a momentary fantasy, and Superman, although enraged and grief-stricken, merely intends to arrest Black and then mourn his wife. Astonished, Black asks him why — and Superman merely replies that neither beating nor killing him would bring Lois back, and would in fact shame her memory. Broken when he realizes that Superman is the genuine article, Black reveals that Lois' death was merely an illusion before killing himself.
Thor may have a bit of a temper and he can be a little aloof, but underneath that, like Wonder Woman, he's got a massive heart. He's a kind, wise man who loves his little brother, is a Friend to All Children and is a god who actually answers prayers. He is, however, also the God of Thunder and Lightning and the Mightiest Avenger Of Them All, and if you piss him off, he will tell you this very loudly just as he is putting his hammer through your face or summoning a personalised hurricane to drop on your head. He has singlehandedly taken down Mangog and Thanos simultaneously, proven Superman's equal and the Hulk's superior, and sent Galactus running for his life.
And then there's his Badass Grandpa future self, King Thor. He may be a bit grumpy (and after what he's been through you can't really blame the guy), but he will not stand for you attacking his favourite planet (Earth), even if it is all but dead and your name is Galactus.
In an actual example, there's the Superman story "For the Man Who Has Everything", in which Mongul traps Superman in a Lotus Eater Dream of a Krypton that never blew up by means of an sentient plant called the Black Mercy. The dream gradually turns into a nightmare as Batman, Robin and Wonder Woman battle Mongul, and when Batman finally frees him from the plant, Superman proceeds to unleash his rage in full upon Mongul, including one memorable scene in which he blasts the tyrant with his heat-vision: "BURN." This was later made into an episode of Justice League, which follows the same exact formula (including "BURN.").
Probably the best Superman example was the famous "Death of Superman" arc. Losing to a monster that took out the entire Justice League with one hand tied behind its back, he comes to a grim decision. "To stop him I'll have to be as ruthless as he is." To which Lois replied with "But he wants to kill, and you CAN'T" also qualifies as a Last Stand, as he truly intended it to be a fight to the finish that would claim the lives of both combatants.
Then there's Alan Moore's Silver Age Superman swan song, "Whatever Happened to The Man of Tomorrow?". The alt-future adult Legion of Super-Villains joins the attack on the historically-doomed Man of Steel killing the empowered Jimmy and Lana as they go. When Cosmic King taunts him to toss them Lois, so that so that they can kill her like his 'other girlfriend' his eyes burn red—the heat vision is a mirror of the pure rage on his face. He burns Lightning Lord's arm, and then, Saturn Queen reveals via telepathy that he's not kidding, and means to kill them all. Their locked-in victory no longer certain, they beat a very hasty retreat to the future.
Notable bit of summary: at one point Superman throws an uppercut. After the "World of Cardboard" Speech. The punch itself seems to cause about three or four meters of shockwave around it.
Let us not forget the Superman story ("The Price," in issue 22 of Superman volume 2) where he travels to an alternate Earth to find that General Zod and his companions have murdered that Earth's Superman and every human on the planet. Zod & Co. taunt Superman about it, believing that they are protected by Superman's code against killing. Superman points out that he is now the sole representative of Earth and, as such, has the responsibility to pronounce a just sentence (and to ensure that the villains don't follow Superman back to his Earth and kill everyone there, too). He sentences them all to death and opens a lead box containing green Kryptonite, directing the radiation at them while using the lead box to shield himself. Superman not only waits until Zod & Co, are dead, waits several minutes more to be sure they're not faking it, and leaves them exposed to the Kryptonite when he departs, just to make triply certain they are dead.
In the second issue of John Byrne's Alpha Flight, the 'til now sweet and innocent Marrina figures out she's an alien. Another of her teammates is still in hospital three issues later...
There was once this nice guy who fell in love with a woman and she was due to have twins. He had superpowers and had been persecuted for being Jewish during the Holocaust, but he was still a pretty cool guy. Then a bunch of people burned down the inn he and his girlfriend were living in. That is roughly when he became Magneto.
For additional fuel, the couple had another child already, who died in the fire, which was set as revenge when the guy insisted on being paid for work he'd done. The arsonist then pretends to grieve for the death of the child.
At a later point in his life, Magneto has found a measure of peace and new love, while working for Mossad. When he wipes out a pet Nazi they keep for information and other uses he fulfills, they make the mistake of killing his girlfriend right in front of him, then taunting him with being invulnerable to his powers due to protective devices they wear. Protect them from direct attacks, yes, from shrapnel, not so much. After this, he sort of loses it for a few decades. Magneto is a wonderful example of why you shouldn't mistreat people who can kill you with their brain.
In issue 42 of Marvel's "What If?" comic, we see an alternate universe where Susan Richards perishes in childbirth due to actions by the villain Annihilus. Driven mad with grief, Reed Richards turns his considerable mental chops from creating gadgets to getting the most violent, terrible revenge he can. He proves to be a far deadlier unhinged genius than Doom ever was, and even causes Namor to tell him to calm the heck down. This being an out-of-continuity tale, it doesn't work.
Another from The DCU: Plastic Man. Although he's normally the team clown, he can get... testy, if pushed. In "The Obsidian Age", he helps the team recover from the psychological effects of time-travel with bad jokes. Then, when Rama Khan sets the Martian Manhunter on fire, Plastic Man uses his own ductile body to choke the dude into unconsciousness ("You like burning? How about the burning on the inside on your lungs right now, like that?") and subsequent brain-damage.
Not to mention the fact that he went toe-to-toe with Fernus The Burning after the baddie in question had already curb-stomped the entire Justice League. Yeah, it was due largely to the fact he was the only member of the team who was immune to Fernus' telepathy, but seeing the Plucky Comic Relief putting the guy who just bitch smacked Superman in a headlock still sends shivers down my spine.
Many people forget that before his origin, Plas used to be "Eel" O'Brien, a career criminal and quite a nasty piece of work. Even he managed to mostly forget, until the League got split into their heroic and secret identities. He pushes the others to undo it because he doesn't want to go back to being that guy.
Often forgotten is that Plastic Man's raw power is immense; as stated by Batman in "Batman: The Dark Knight Strikes Again".
"There is no end to what he can do. Whatever he imagines... whatever flickers across his subconscious...he becomes."
Captain Marvel is basically the nice guy all other comic book nice guys aspire to be, to the point that he compares favorably even next to Superman. One story featured him getting into a fight with a villain that destroyed a homeless shelter, and not only did Cap talk the villain down from continuing the fight, but then further talked him into repairing the damage to the homeless shelter, and got the villain to leave peacefully. Billy then spent the next day serving food at the homeless shelter. He's just that nice a guy. But at the same time, even he has his limits. When a group of hired killers murdered Billy Batson's best friend, Captain Marvel stormed into the police headquarters and grabbed the lead killer by the head, and started speaking to him in a Tranquil Fury.
Captain Marvel: "Who hired you? Tell me now. Or I'll crush your head ... Then I'll walk downstairs to the holding cells and ask your partners ... I'll bring your dead, headless body with me ... and then they'll tell me. So ... for the last time ... who hired you?"
The next scene showed Captain Marvel hovering outside Dr. Sivana's office, revealing the assassin Sivana had hired had told Cap everything he wanted to know.
Colossus from the X-Men is normally the team's Gentle Giant, except for when Nightcrawler and Shadowcat are gravely injured by Riptide and Harpoon during the Mutant Massacre. On a single page, Colossus snaps Riptide's neck and swears to do the same to Harpoon.
Colossus: "HARPOON! Make peace with your gods, little man! YOU. ARE. NEXT."
Or in Days of Future Past when Wolverine and Storm are killed. One panel focuses on Colossus' grief-stricken face. The next panel shows us a Sentinel getting thrown through a skyscraper.
Another example was an early appearance of Jubilee. Upon seeing Wolverine seemingly killed at the hands of Mandarin and a brainwashed Psylocke, she lets out a Big "NO!" and proceeds to blow up Mandarin's castle. Once the dust settles, a bewildered Jubilee simply says, "Did I do that? And, like, do I want to do it again?"
Dust from New X-men is normally a quiet peaceful person who doesn't like to fight but when mutant hunters started killing her friends and loved ones all bets were off.
Spider-Man. Oh sure, he's a little hot-headed and reckless at times, but for the most part, he's a good guy. He's also one of the few (remaining) (Marvel) superheroes who makes a rule of never killing. But if you mess with him, and I mean really mess with him... well let's just say if Norman Osborn's glider hadn't made a Goblin Kebab out of him when he killed Gwen Stacy, even Spidey doesn't know how far he'd have gone.
And he became downright scary in Back In Black. I doubt Kingpin will ever mess with him again. And let's not forget the whole "throwing a Jeep through a wall at the sniper" thing.
And then there's the What If?: Back In Black issue. He beats the crap out of Iron Man (multiple times) and murders Kingpin.
At one point, a sleazy tabloid photographer makes the completely untrue claim that Mary Jane's been having an affair with Tony Stark. Logan makes the stupid mistake of making comments about MJ just after Peter's heard about the article. We get one panel of Peter looking extremely pissed, then the next panel, Luke Cage and Jessica Drew are looking at the hole in the supposedly unbreakable glass where he just pitched Wolverine through the plate window. From several stories up.
At one point, a future version of Iron Man from 2025 comes back in time to get the biometric signature of a child who would grow up to be a terrorist in the future (he needed it to defuse a bomb). Spider-Man, not knowing what's going on, interferes. Rather than explain the situation, Iron Man fights Spider-Man and ruthlessly injures the child in the process (which incidentally is the reason he grows up to hold a grudge against Stark Industries). Since Iron Man is supposed to be a hero, Spider-Man flips out. He beats the future-tech Iron Man to a pulp with his bare fists in a no-contest ass-whupping.
Peter David's famous "Death of Jean DeWolff" storyline shows what happens when Spider-Man gets pushed too far, and it's not pretty. The story centered around the murder of one of Spidey's few friends on the NYPD by the psychotic "Sin-Eater", who went on to kill a priest who opposed capital punishment, a judge who "coddled criminals", and an innocent bystander who got clipped when Spidey dodged a shotgun blast. At the same time, an elderly gentleman from Aunt May's boarding house is viciously mugged by three punks, whom Matt Murdock manages to get bail for. Spidey is shown getting angrier and more aggressive towards criminals as he tries to track down Sin-Eater, even putting a drug dealer in fear of his life by taking him to a bar and making out that they're buddies, in order to get some info (the guy later goes into Witness Protection), earning him a What the Hell, Hero? from Daredevil. It comes to a head as Spidey and Daredevil discover that The Sin Eater is actually Stan Carter, the detective in charge of the Sin-Eater case, whom Spidey had been developing a friendship with, and that he has gone to J. Jonah Jameson's penthouse to kill Jameson (JJJ's out of town, but his wife Marla is there, along with Betty Brant-Leeds, one of Peter Parker's oldest friends and a former girlfriend). Sin-Eater is about to kill Betty when Spidey bursts in, snaps his shotgun over his knee like a twig, and smacks him around, causing Carter to snap back to sanity for a moment and try to apologize, which pisses Spidey off even more, as he continues to viciously pound Carter, even after he's unconscious. Daredevil tries to intervene before Spider-Man kills the guy and gets punched out of a window for his efforts, and then barely defeats the raging, out-of-control Spider-Man. Later, as the cops try to get Carter into a wagon to Ryker's, they're swarmed by an angry mob looking for some vigilante justice. Daredevil jumps in to stop them, whilst Spidey stays where he is and watches, until Daredevil calls out to him by his first name, and snaps him back to his senses. Peter David brought Carter back in a later storyline - partially deaf, with a stutter and a limp as a direct result of what Spider-Man did to him.
There was also the Burglar who killed his Uncle Ben. After getting released from prison, Peter begins relentlessly pursuing him. When the guy asks why Ben's death matters so much to him, Peter whips off his mask and tells him he killed his only father figure in life. The Burglar was so scared of Peter, he had a fatal heart attack.
There was also a What If? one shot that showed Peter beating the Burglar to death.
In a more recent example, we have the Grim Hunt arc where Kraven's family decided it was a good idea to start systematically killing off Spider-themed superheroes to get to Peter. It really wasn't, seeing the way Peter went off on them was pretty disturbing in a way.
After Grim Hunt, Doctor Octopus hires just about every villain in New York to capture Osborn's baby. After the newborn is seemingly killed, bad guys are scared enough to try to turn themselves in to the police to get away. It doesn't help.
In an issue called "Point of View," Spider-Man finally decides he's had enough and smashes into J. Jonah Jameson's office, sealing the publisher in and scaring the living crap out of him, so that Peter can satisfy himself by putting his fist through Jameson's face. The only thing that finally holds Spider-Man back is Jonah himself, who tells Spidey that either he really is the menace that Jameson has crucified in a hundred editorials, or else Jonah has convinced him he is, because he's sure acting like one. "And frankly, masked man, I didn't think I was that good a writer."
(pause; Spider-Man lowers Jameson to the floor) Spidey: You stink, Jonah. You really do.
The denouement of the "The Book of Ezekiel" implies that Peter Parker was chosen to become Spider-Man for this specific reason.
Spider Totem: Who could be a better hunter then one who had been prey? Someone who would be driven to fight back against the dark forces sent by the world, who would never stop, even though they were bigger and more and perhaps even stronger than he was. Because once having been prey, he would never allow himself to become such again. Would never surrender. Would take death before submission.
In one storyarc, Spider-Man is tasked with protecting a newborn child and ends up fighting the majority of his rogues gallery. When he is tricked into thinking the baby has died, he goes on a rampage. At one point, a group of c-list villains are seen actually turning themselves in before Spider-Man finds them, figuring that being arrested is a better fate than whatever beating Spidey has to offer.
During the Secret Wars, Spider-Man delivers a savage beatdown to Titania after she humiliated She-Hulk. Titania, who by all accounts was stronger than Spider-Man, was so traumatized by her defeat that she developed a longstanding phobia of the webslinger.
You can do a lot of things to that nice attorney Matt Murdock... just don't kill his father, or girlfriends, or the people in his town, or else Daredevil may, if you're lucky, break your back, beat you to a pulp and throw a speech, engrave a bullseye in your head with a sharp stone or paralyze your legs permanently with a sword. If you really make him go over the edge, he will come back for you. To kill you. With ninjas.
Mrs. Jean Grey-Summers, also of the X-Men. Despite the fact that she has an occasional temper and she is most known for hertragic sacrifice in the Dark Phoenix Saga, she is mostly a very compassionate and loving woman who cares for just about everyone around her. Mostly is the key word here as when she found out that Rich Bitch Emma Frost had a telepathic affair with her husband Scott, she broke into Emma's mind and humiliated her severely. Also, when a team of mutant organ harvesters known as the U-Men attacked the X-Mansion, she used her powers to make them vomit and defecate in their suits before she tore them off and made them flee, all while uttering one of the greatest threats ever put in a word bubble if they dared to hurt anyone again.
Jean Grey: I don't want you to get hurt, but you have to understand, the more you annoy me the more I can't help thinking about deconstructing you, molecule by molecule, memory by memory until there's nothing left of you but screaming, traumatizedATOMS.
And while we're at it, let's not forget her husband, Mr. Scott Summers, a.k.a. Cyclops. While perhaps not as naturally nice as Colossus, Scott spent much of his life trying to be a nice guy, even when it was grossly apparent that no one appreciated his efforts (except for Jean and maybe Professor Xavier), but more than once, Scott has violently lashed out at those who dared push him too far. A number of supervillains, such as Mr. Sinister, found themselves on the wrong end of more violent than usual optic blasts once they incurred Scott's wrath. Wolverine himself, the loner who usually got off on harassing Cyclops, has also felt Scott's fury: in the 1970s, after an issue where the X-Men fought a villain who had brainwashed former X-Men and as well as Scott's brother Havok and Havok's girlfriend Polaris, Scott violently backhanded Wolverine with a closed fist when the clawed one yelled at him over the turn out of the battle. During Joss Whedon's run on Astonishing X-Men when Scott started officially dating Emma Frost, Scott violently blasted Wolverine out of his bedroom when the clawed one dared stick his nose too far into Scott's personal life.
Let's not forget Squirrel Girl. Sweet disposition, friendly, a noticeable lack of Angst and apparently the loser in the Superpower Lottery. She's taken down a wide swath of A-list villains.
Recently in New Avengers, we saw her sparring with Wolverine, where she pretty much whopped his ass. She was not exactly mad, but there was some bit of tension there, so that just makes one ponder more on how she'd be when angry. Then not long afterwards, after being hired as a nanny by Luke Cage and Jessica Jones for their daughter Danielle, cue murderous Nazis in killer mecha, destroying everything and killing everyone in sight, threatening the safety of Danielle. They never knew what tore them apart.
Clearbrook in ElfQuest is usually the calmest, most rational elf you could hope to meet, but after her lifemate One-Eye is killed by trolls she briefly becomes a trollicidal berserker during the subsequent elf-troll war. Definitely not played for laughs, as the other elves know her rage could destroy her.
Redlance's desperation-move of using his benevolent plant-shaping to make a troll's spear sprout thorns straight through its wielder's hands also qualifies.
Vice versa as well. The opening of volume 2 sees anti-superhuman terrorists threatening Hulkling with a bomb. When Wiccan goes off, everyone's first thought is that the bomb did. After the dust settles, the Avengers give their younger counterparts a talking to about how Billy is a person of mass destruction every bit as fearsome as his mother.
There was once this really nice guy, who had finally come back from the Vietnam War and just wanted to spend it with his family. He was an ordinary retired Marine, who loved his wife and two children wholeheartedly. Then, he took them to a picnic in Central Park....
There was once a young boy, Frank Castle, who was friends with an older Marine and witnessed the hold the local gangsters had over the neighborhood. After his friend's sister, who liked him, was raped and she kills herself, and he listens to his parents saying about how afraid everyone is, Frank takes his father's gun to kill the punk who raped his girl. But he never got the chance, as the girl's older brother set the punk on fire first.
DCU's Miss Martian. M'gann is notoriously sweet, charming and really just the kind of person that likes cute puppies; however, she is actually a member of the stupefyingly powerful White Martian race, and although she isn't a bad guy in the slightest whether or not she'll succumb to her baser instincts is always up for debate. In later issues, she fights and then merges with an evil future version of herself who apparently committed and instigated such unspeakable crimes against humanity that the entire White Martian race was captured and enslaved because of her.
M'gann's teammate Kid Devil is also one to beware. While Eddie projects the image of a loveable loser, he hides a lot of anger from constantly being underestimated by villains and his peers. In one instance, while drugged up by the Dark Side Club and thrown into a death match with Hardrock (a teenage Thing), he's beaten to the point that he suddenly loses it, turns the fight around in just a few punches, and almost kills Hardrock by ripping his jaw off. It takes Miss Martian to talk him out of it, reminding him that he's a good guy. The second instance comes when Eddie and Blue Beetle are tracking down supervillain Shockwave, whom had mockingly called Eddie Beast Boy the first time they fought. They split up to look for Shockwave, but he's in the wrong city. Upon finding out, Eddie loses his temper, furious that he's been upstaged by Beetle yet again. In his rage, he creates a portal for the first time and teleports to Salt Lake City to viciously beat up the villain and melt his armor.
Kyle Rayner is often regarded as one of the most dangerous Lanterns not necessarily because of his temper or wrath, but because when you piss him off, he will wipe the floor with you in the most humiliating fashion imaginable (how would you like to be man-handled by an adorable Teddy bear or an Engrish-speaking Magical Girl?). Kyle doesn't get angry, he gets creative.
Kyle freaks out the rest of the Justice League when he casually announces he can split atoms with his ring. And that that isn't hard, what's hard is containing the blast.
He is, of course, messing with their heads with that statement, though the truth is even worse: this is the guy who contained an artificial supernova with his ring during the climax of the DC One Million crossover event. The last GL ring (which is what it was at the time) was not known as the most powerful weapon in the universe for nothing. Combine it with Kyle's imagination and, well...
Of all the members of the Bat-family, you wouldn't expect Nightwing to be the baddest, but he has proven he is. Just ask Blockbuster. He also beat The Joker to death (of course, he is later revived by Batman).
Speaking of Batman, it's not a good idea to push the Commissionor or his daughter. They may be calm and compassionate people, but they don't take kindly to threats and are mental juggernauts.
Oracle: Every database, every security camera, every police band has my fingerprints on it. Every moment you're out the noose pulls tighter. ...NOBODY messes with my partner.
Jefferson Pierce is a wonderful, wonderful man and one of the nicest people you could ever hope to meet, as well as an avowed pacifist, but push his buttons and he will show you why pissing off one of the most powerful metahumans in existence is a bad idea.
Dr. Magnus, creator of the DCU's Metal Men, is normally a fairly timid guy. As 52 showed us, though, after being kidnapped, forced to recreate the Plutonium Man, pushed to the breaking point by Chang Tzu, and deprived of his medication, he battles, and defeats, a member of the Great Ten with nothing but a group of makeshift six-inch Metal Men and a particle wave weapon. He even frightens off other Mad Scientists, after they had previously been almost eager to confront the JSA. As he puts it: "I DO CRAZY THINGS WITHOUT MY MEDS!"
There's also Tin. Tin is a shy, meek little milquetoast with a pronounced stammer and a very slight physique. However, he's got something to prove and is repeatedly shown to actually be the bravest of the Metal Men when it comes down to it. Also, he will fuck you up given half a chance and proper motivation.
Bruce Banner himself isn't exactly helpless either. Word of God confirms that his intelligence is on par with Tony Stark or Reed Richards; he's been able to avoid the authorities countless times and was able to hold his own in a few fights without turning into the Hulk. If you are dumb enough to piss him off, heWILLsmashyou.
All the above is explored in the 'Banner & Son' storyline, which sees Banner temporarily unable to transform into the Hulk. He still manages to take down villains like Juggernaut and the Harpy by being a Gadgeteer Genius (and with a little help from Skaar). In fact, Norman Osborn considers Banner to be a far greater threat to him than the Hulk ("The Hulk is purely reactive... Banner thinks he's some sort of hero") and exposes Banner to a substance which will accelerate the reacquisition of his Hulk powers.
During this time, he also muses whether the Hulk is there to protect him from the world or to protect the world from Banner.
Hulk himself actually quite a timid guy... if you leave him alone. Sadly, the usual response is... well, doing what the Hulk does.
Bruce killed his own abusive murderous father in a fit of rage (in self-defense) in his backstory long before he was hit by the gamma-bomb. He's always been dangerous.
Wedge Antilles is a self-possessed heroic Ace Pilot who is about as good as you can get. He's got far less of a temper than most of his pilots, is loyal to his friends, and is unusually welcoming of people who have defected to his side from the Empire. But in Mandatory Retirement, one of his pilots died rescuing a high-ranking Imperial defector. Said defector objected to being transported in the same shuttle as "animal filth"(the dead Rogue happened to be nonhuman). Wedge responded by grabbing the man by the collar, doing a two-handed Neck Lift, and saying "Don't make me go Vader on you. Ibitsam was a pilot and a friend and she died to save your sorry hide."
Betty and Veronica, of Archie Comics fame, can get VERY steamed at Archie, usually relating to dating issues, but also if he's being an idiot.
THE perfect example: Ozymandias from Watchmen. "Hitler was a vegetarian."
Who can forget Nite Owl? Adorkable, compassionate, loves animals (owls mostly), always willing to help people (saving people from that burning building was all his idea), donned the hood because he was a fan boy of the original "Big Bouncing Boyscout," but he can break your arms, legs, and several ribs if you try to mess with him.
Professor Calculus. While he's generally very pleasant and friendly (if extremely deaf), that does NOT mean you should insult his work or call him a goat - see Tintin Destination Moon for full details.
Tintin himself. He's a good-natured young man with a high sense of morality... but he can still punch you through a wall if you deserve it.
Three examples from Paperinik New Adventures: Paperinik is usually cheery, fun and wisecracking, Urk is a nice and tranquil guy, if a little big, and the alien Xado is shown in flashbacks as a very nice alien Hot Scientist who really loves dancing. But keep in mind that Paperinik was originally born as Donald Duck's way to vent frustrations and his full superhero name is Paperinik the Devilish Avenger, Urk is strong enough to accidentally break people's ribs with a hug, and outside flashbacks Xado is now Xadhoom, a Physical Goddess bound to fully exterminate the Evronians (the aliens responsible of the destruction of her homeworld and her people) in the most painful way she knows while asking them if they want to dance.
Klara Prast from Runaways is this. Having been wrenched out of the early 1900s, when kids were usually spanked into submission, she's as sweet and polite as can be. But don't you dare upset her, or else she'll sic the vines on your ass, or raise a forest under your ass. Or both. And if you should happen to remind her of her husband (i.e. the drunken, misogynistic, pedophilic son of a bitch who used to beat and rape her before the Runaways rescued her), she'll not only kick your ass, but she'll also pretty you up by putting flowers in your hair, so that everyone in the holding cell will know that you got your ass handed to you by a little girl.
Bayarmaa, Oz's wife in season 8 of Buffy the Vampire Slayer, will offer you a place to stay, instructions on how to deal with your inner demon and a seemingly neverending amount of butter tea. Threaten her loved ones and she won't hesitate to rip your throat out with her teeth.
Death and Delirium of the Endless in The Sandman have shades of this. Delirium, usually fairly happy and friendly, hands out Disproportionate Retribution like candy and, although totally insane, at one point was able to pull herself together and coldly and somewhat threateningly tell Destiny to stick it. As for Death, she terrified Desire into silence with a single, vague threat and made the (up to then) unstoppable Kindly Ones take on a scared, humbled tone and then back off just by shouting at them.
Hawkeye. He's a man who hates killing, but if he finds out that his apparently returned wife is actually another Skrull imposter? You don't want to be a Skrull he meets.
In American Vampire , Pearl Jones is a nice, unassuming young woman who just wants to live in peace with the man she loves. Threaten that man, though, or push her hard enough, and it doesn't matter if you're her ex-BFF or the vampire who made her a vampire in the first place, she will demonstrate just how dangerous her new strain of vampirism really is.
In The Transformers, it's Ratchet of all 'bots who comes the closest to putting Starscream down for good, when Starscream's trying to kill him and complaining about it not being "fun". The only reason Ratchet doesn't succeed (as far as we can tell) is because the ship crashes.
Mighty the Armadillo is the same way. He's the nicest of the Chaotix, but harm his friends, especially Ray the Flying Squirrel, and he'll tear you a new one after he tears apart whatever's keeping you from him.