Alpha Flight: In the second issue of the first series, the 'til now sweet and innocent Marrina figures out she's an alien. Another of her teammates is still in hospital three issues later...
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 her vampiric sire, she will demonstrate just how dangerous her new strain of vampirism really is.
Archie Comics: Betty and Veronica can get VERY steamed at Archie, usually relating to dating issues, but also if he's being an idiot.
Though Batman himself usually isn't considered "nice" (which he himself has stated on more than one occasion), he always manages to toe the line and not break his no killing rule. But there have been a few occasions where he has been pushed too far and has come dangerously close to doing just that.
Arguably the best example is in Batman: Hush, when it appears as though the Joker has killed Tommy Elliot (though this "Tommy" would later revealed to be Clayface, with the real Elliot, acting as the eponymous villain Hush, having manipulated the whole situation from afar.) It appears as though this is the time that the Joker has pushed Batman to his breaking point, as he begins attacking Joker much more brutally that he usually does, like, to the point where the Joker himself (who usually enjoys it when Batman beats him up) begins begging him to stop and adamantly insists that he didn't kill Elliot. Both Harley Quinn and Catwoman try to stop him but are unable to. We actually see Batman contemplate which way to actually kill the Joker, debating whether he should crush his windpipe, push his ribs through his lungs to rupture his heart, or even repeatedly smash his skull into the pavement. It's only after Jim Gordon arrives on the scene and fires several warning shots at him and actually manages to reason with him that he relents.
Batman himself is actually shocked at how far he went. He flashes back to the deaths of his parents, and about how he swore he'd rid Gotham of the evil that took their lives, and reflects that he had just come dangerously close to "becoming a part of that evil."
It's not a good idea to push the Commissioner Gordon or his daughter Barbara "Batgirl" Gordon. They may be calm and compassionate people, but they don't take kindly to threats and are mental juggernauts.
James Gordon: It's been 15 years since I've had to take out a green beret. Even so, he deserves a handicap.
While most of the Robins have notorious and well known tempers, including Nightwing, Tim Drake has a well earned reputation for maintaining a level head, because he funnels his anger into quietly and secretly plotting the thorough demise of those who piss him off. A couple of bad futures show just how dangerous he could be if pushed past his breaking point. The danger he poses is compounded by his flexible view on the family's no killing rule, even when he considers himself to be sticking to it he's willing to blow up the place his enemies are located at as long as he tells them they ought to leave while he's doing so. He's also set two sets of assassins against each other to fatal results after both got on his bad side.
Black Lightning: 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.
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 never-ending amount of butter tea. Threaten her loved ones and she won't hesitate to rip your throat out with her teeth.
Captain America, though usually stoic and serious, is known by those who are closest to him to be the nicest guy you could ever meet. However, the Red Skull has this knack to really rile him up.
Circles: Paulie is the sweetest man, but he can get extremely angry if you set him off.
ClanDestine: Albert Destine is a pacifist with Healing Hands. When an Inhuman criminal kills his brother Thaddeus, he finally snaps, falls into a rage, and kills the murderer by using his healing powers as a weapon.
Dan Dare is as generous, kind and loyal a man as you could ever want to meet. Give him a reason to show you how good he is with his fists, though, and he will very happily oblige.
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.
Lahr the Gelfling from the The Dark Crystal prequel manga. He starts the story as a laid-back shepherd. Then Garthimdestroy his village, kidnap all of his friends and family, and break his flute. Lahr reacts by stabbing a Garthim to death with the broken flute and wields the flute like a sword for the rest of the book He later helps convince a nearby Gelfling village to take up arms against a Garthim swarm. This when Gelflings were an artistically inclined, peaceful race who had believed Garthim to be indestructible.
Donald Duck is best known for his Unstoppable Rage, during which he becomes a One-Man Army. When he's not angry, he's a cheery, fun and wisecracking guy, and a good Parental Substitute— in fact, the surest way to get him angry is to threaten his nephews.
Reginella, queen of the planet Pacificus is a sweet and demure woman, if a bit sad due the duties of ruling over her planet. Whenever her planet is in danger and the usual means don't work, her first reaction is to summon Donald and have him go on a rampage on the threat, or otherwise use his help to annihilate it, and escalates from that, with her final weapon being nightmare-levels of Mind Rape (we don't know if Vampirione ever woke up from the coma)-and the latter would actually be her first solution if it didn't threaten her own life due the strain.
During her debut story she's seen delivering a Death Glare to a Pacificus that went mad with anger, before leading her people to forcefully calm him down with their psychic powers and disapproval. In light of the fourth story, when she first used her full powers, that moment becomes much more terrifying.
Three more 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 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.
Huey, Dewey, and Louie are probably the kindest members of the Duck family. In a possible future where their uncle had disappeared (actually sent forward in time) they took over as Paperinik the Devilish Avenger, and were almost as terrifying and efficient as the original, to the point the only crime left in Duckburg was the Beagle Boys-and even then, they had to pretty much form an army. Army that fell in ten minutes when Paperinik arrived in their time and led them to the Beagle Boys' hideout.
Gus Goose is one of the kindest members of the Duck family, and by far the laziest. Threaten his loved ones or his lunch is the quickest and surest way to find out what he did to that poor bear that Donald saw running from him.
Surprisingly enough, Magica De Spell. Turns out she's quite friendly, nice and even funny to her friends, no matter how vile she can be to her enemies and anyone in her way.
Clearbrook 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 is usually the sweet, gentle one who is left to watch the kids -which is why he was there when the trolls attacked. His desperation-move of using his benevolent plant-shaping to make a troll's spear sprout thorns straight through its wielder's hands qualifies him for this and Papa Wolf. It's implied he did much more than just injure a troll's hands, and he goes into a catatonic state afterwards at what he's done. Fortunately, elves have Sending to reach into a person's mind.
Fantastic Four: A bit of a recurring theme in the various Marvel universes seen in several issues of What If? and the Ultimate universe is the fact that when Sue Storm and Reed Richards are not a couple he becomes a villain. 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 — who, in what is possibly the wisest move he ever makes in any universe, decides not to take advantage of the situation and just decides to quietly stay out of Reed's way on this one. Reed's increasingly irrational actions even cause Namor to tell him to calm the heck down. This being an out-of-continuity tale, it doesn't work.
Gaston Lagaffe: Oh my goodness!!! Gaston Lagaffe, the titular character is canonically the nicest character in the entire franchise, but sometimes, he snaps.
Yves Lebrac as well, when you mess with his artwork and tools.
Green Lantern: 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...
Harley Quinn: despite being a villain (or, really, more of an Anti-Hero now) is surprisingly nice to people as long as they treat her with respect. Anyone who threatens or insults her (or does other things she hates, such as animal cruelty) will usually get a No-Holds-Barred Beatdown at the least.
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.
The Incredible Hercules is basically a big drunk teddy bear unless you're looking for a fight, then he becomes a big drunk grizzly bear.
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. In one of the Avengers vs. X-Men issues he proves that he is smarter than Stark, humming afterwards "Banner is the smartest one there is."
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.
Part of Bruce's backstory is that he killed his own abusive murderous father in a fit of rage (in self-defense) long before he was hit by the gamma-bomb. He's always been dangerous. The gamma rays just gave his rage a form.
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.
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.
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 rival, and sent Galactus running for his life.
And then there's his old 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.
Bombo from Monster Allergy is a big and nice monster who really likes to eat (and in fact got in trouble with monsters' justice because he was too much of a Big Eater), and a bit of a fool. Then in the sequel Zick's out-of-control Dom attacks Zick... And Bombo, for the first time ever, gets furious and eats it. Made scarier because in the act he morphed All-Eating Bonz, a race of monsters that looks like Bombo but has no limit on eating and is prone to go on rampages where they eat everything and everyone in sight.
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).
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.
Papy PK from Noob generally acts like a sweet old man, but has been shown to to wipe the floor with the few players he's been seen fighting so far.
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.
There's 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 peoples' 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.
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... and since then, he walked away wearing a skull.
The Punisher MAX: 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.
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.
Death and Delirium of the Endless in The Sandman (1989) 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. Also, y'know, she's Death.
Captain Marvel, also known as "Shazam", 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.
Marvel supervillain Shocker is well-known as one of the friendliest, nicest people around despite being a supervillain. He's extremely professional and laid-back, refuses to hurt innocents, and never holds grudges or feuds with superheroes. However he's also a veteran supervillain, an expert fighter, wields a pair of sonic gauntlets that can level a small building if powered up enough, and is extremely dangerous when provoked. In The Superior Foes of Spider-Man after Boomerang insults him one time too many, Shocker snaps and proceeds to effortlessly beat the shit out of four other supervillains at once. A few issues later he blasts the freaking Punisher half a mile away with a single shot.
The Silver Surfer is one of the most pure, gentle, and kindhearted heroes in the entire Marvel Universe. He’s also one of the most insanely powerful heroes, wielder of the Power Cosmic, and capable of battling gods like Thor on even footing, making it a very bad idea to piss him off. One version of him from a Bad Future took on two versions of Thanos, a cosmically-enhanced Ghost Rider, and the Hulk all at once; he completely dominated the battle, pummeling both versions of Thanos, killing Ghost Rider with one hit, and defeating the Hulk by standing still. Hulk mangles his own hands by punching the Surfer repeatedly. Willfully Weak doesn’t even begin to describe the Surfer’s situation. This is, after all, a guy who used to help Galactus murder entire planets.
In Sonic the Hedgehog (Archie Comics), the titular character, when pushed to his limits, will break limits. And walls. With your face, as Silver will attest to.
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.
Sonic the Hedgehog (IDW) also does this. The heroes, including Sonic himself, are nice guys and gals overall, but if you push them too far, you're in for a world of hurt.
Cream the Rabbit provides the series' best example of this trope during the Metal Virus Saga. Cream is one of the nicest and cheeriest of all Sonic characters, but she can't stand cruelty, and after Dr. Eggman's Metal Virus bioweapon shatters her cutie, she's in no mood for the bad guys' crap. Eventually, the heroes' only hope of stopping the plague is a truce with Eggman himself against his rivals, the Zeti, who quickly get on Cream's bad side, big time. One Zeti, the sadistic narcissist Zeena, finally drives Cream to her Rage Breaking Point and pays the price, as does the Zeti leader, Zavok, later on. It goes to show that although Cream truly is a Nice Girl and prefers resolving things peacefully rather than violently, she's not afraid to assert herself and fight when crap hits the fan.
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.
There's the What If?: Back In Black issue:
He beats the crap out of Iron Man (multiple times) and murders Kingpin. This reflects how in the main timeline, Spider-Man warned Kingpin that if Aunt May died of her wounds (this rampage was started because one of Kingpin's assassins shot her while aiming for Spidey), that he'd be back to kill Kingpin.
He became downright scary. He broke into a prison just to beat Kingpin to within an inch of his life, and threw a Jeep at a sniper. Through a brick wall.
Heck, the mainstream Back in Black isn't too shabby on this front either. When Spider-Man breaks into the prison to confront him, Kingpin has set up a prison fight between them, confident that he can go one-on-one with Spider-Man. He very quickly learns far too late that these are mere pretensions when Spider-Man, deeply unamused by his aunt being shot and in no mood to keep the kid gloves on, utterly wipes the floor with him, leaving him utterly humiliated him in front of the entire prison population. He then makes two things absolutely clear: one, that this is far from the most horrific treatment that Kingpin will suffer at Spider-Man's hands if / when Aunt May should die from her injuries; and two, that anyone else that thinks attacking Spider-Man through his loved ones is a good idea will answer to him as well. Realising how easily he could tear any of them apart having practically crippled Kingpin without breaking a sweat, the other prisoners get the message.
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. Wolverine can survive that just fine, but it still would've hurt like hell.
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.
The "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.
Quick tip for villains - if Spider-Man is fighting you, make sure he's joking around. If he isn't, then congratulations — you've succeeded in really pissing him off. Remember, this guy once went toe-to-toe with a Herald of Galactus and won. He's survived fights with the Hulk and the Juggernaut. He can bench press buildings, dodge bullets before they are even fired, and has a genius-level intellect that even Tony Stark and Hank Pym are a little jealous of. All that joking around is him working around his nerves and limiting his strength against normal, fragile, human crooks. What do you think is going to happen to you if he decides to dedicate all that strength and intelligence into destroying you?
This aspect of his reputation is what made the Superior Spider Man so frightening to Spidey's usual rogues. Nobody knew that it was really Doctor Octopus in the web-slinger's body—all they knew was that Spider-Man's usual jokes and jibes were gone, and he was dealing far more harshly with even street-level criminals. To the criminal element, it appeared that Spider-Man had finally snapped and showed no intention of going back to his old self, which had many criminals running scared.
Furthermore in Superior Spider-Man, in one issue, Doctor Octopus, unfamiliar with Peter's enhanced strength, punches an enemy (Scorpion to be exact) so hard it tears their jaw off. Doc Ock has the sudden realization that Spider-Man has been holding back throughout the entirety of their fights. Peter could have easily killed him at any point, and Doc Ock was only spared a violent death because of Peter's self-control.
Played for Laughs in Amazing Spider-Man Annual (2014) when Ruby Thursday remarks to a group of villains that "As long as Spider-Man cracks jokes, the world is all right". Cue Spider-Man hanging above their heads, and as he remains eerily silent as they try to prod him into making quips, Ruby says "Uh-oh." He then quickly beats them, and after S.H.I.E.L.D. agents come to arrest them he quietly says "You're welcome." after being thanked, revealing he just had laryngitis.
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.
In Origin of the Species, Spider-Man is tasked with protecting the newborn child of Harry Osborn, whom Doctor Octopus is trying to kidnap. He ends up fighting the majority of his rogues gallery after Doc hires them to help out in the scheme. When he is tricked into thinking the baby was killed, he proceeds to go on a rampage, beating half of the villains within an inch of their lives. At one point, a group of villains are seen actually turning themselves in before Spider-Man finds them, figuring that being arrested is a better fate than whatever Spidey has in mind for them.
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."
(Beat; 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.
During the Secret Wars (1984), 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.
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.
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.
Not long after whupping Wolverine, 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.
In her solo series, she's nice enough to befriend the likes of Galactacus and Kraven the Hunter... but remains absolutely no slouch in combat, and all the superheroes and villains she's befriended can be trusted to come to her aid.
Is important to know that Squirrel Girl plays by a different set of rules: in her Universe conception The Good Guys Always Win, she has full body Plot Armor and may or may not be a Reality Warper. So yeah, you better not enter in her definition of "Bad Guy".
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."
Supergirl is usually a nice, friendly woman. Unlike her cousin, she's headstrong and a little short-tempered and Hot-Blooded. When she gets real angry, it isn't pretty.
During the Crisis on Infinite Earths, the Anti-Monitor — a being who eats whole universes — almost kills Superman. Enraged, Supergirl attacked him, broke his supposedly indestructible armor and nearly killed him.
In the post-Crisis storyWay of the World, Kara flew Clayface up to the edge of the atmosphere and dropped him after he got her pissed off.
In Red Daughter of Krypton, Lobo pressed Supergirl's berserk buttons over and over again until she exploded and pummelled him.
In Elseworld's Finest: Supergirl & Batgirl, Supergirl is nice, kind, polite and good-natured. She even apologized to Batgirl for firing back after Batgirl insulted her and her family. Still, when she discovered that Lex Luthor murdered her baby cousin, she wanted to kill him. And she damn near killed him, but Batgirl managed to talk her down from crushing his skull.
Bizarrogirl: Dr. Light is usually a nice, good-natured woman but being punched out by Bizarrogirl really got her pissed. Cue energy blast from above.
In For the Man Who Has Everything, Mongul traps Superman in a Lotus Eater Dream of a Krypton that never blew up by means of a 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:
Mongul: Happy birthday, Kryptonian. I give you oblivion. Superman:Burn.
The Death of Superman: 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.
Whatever Happened to the Man of Tomorrow?: The 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.
Supergirl Adventures Girl Of Steel: When Brainiac states he will still win, since Kal and Kara are weakened, Superman retorts a little pain will not stop him because Brainiac has gotten him absolutely pissed off.
In The Supergirl Saga, he travels to an alternate Earth to find that General Zod and his companions have murdered 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, he 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.
Subverted in What's So Funny About Truth, Justice & the American Way?. That story 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 while also revealing his secret identity. 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 he uses his psychic powers to remove the knowledge of Clark being Superman before killing himself.
Perhaps the most perfect explanation of the trope comes in "Foul Play" (Superman 185, 2002), where Major Force (Trope Namer for Stuffed in the Fridge, by action) taunts Superman for being a "Boy Scout" who is "too polite" to do a real job like him. Superman very quickly proves him wrong by words and also, very calmly, melting the metal flesh from his bones (an act which Superman states he knows won't kill him, but still, damn).
Superman: That's a common misconception. The boy scout thing, Major. And being polite. Of course I'm polite to people. Good people. But people like you? People like you frustrate me. People like you I'm not polite to. Are we clear, Major? (The major cannot respond. His mouth is melted shut) Yes. I'm sure we are.
Superman vs. the Amazing Spider-Man: Peter Parker flies off the handle when he becomes fed up with Jonah and tells his boss exactly what he thinks of him. Later Spider-Man goes berserker when MJ vanishes, and he takes on Superman. And he annoys Superman to the point that Supes momentarily forgets to pull his punches and nearly kills Spidey.
Superman: Birthright: Issue #6 has a scene where Superman stops a school shooting, then tracks down and threatens the man who illegally sold guns to a pair of angry teenagers. As in, Supes shoots a bullet at the guy and stops it just inches from the man's face.
Superman: Brainiac: Brainiac presses all Superman’s berserk buttons one after the other until Superman loses his temper and punches him out.
Superman: Inflicting pain on others doesn't seem to bother you. It bothers me. Usually.
In Superman's Return to Krypton, Jonathan Kent, who is usually a calm and nice lad, angrily decks a crook who is trying to make off with stolen money and kidnap Martha.
The Plague of the Antibiotic Man: When Superman and Supergirl burst into Amalak's spaceship, the Man of Steel is definitely in a bad mood -what with an incurable plague spreading, Lois falling sick, and him being fooled into believing he killed someone-, so he recommends Amalak does not try his patience further:
Amalak: What is this—? Two Kryptonians? Perfect! Today I am twice blessed! Superman: Don't get me mad, Amalak— or the only blessings you'll be in for are last rites!
Let My People Grow!: When Brainiac gets hit by his own size-changing ray, Supergirl flatly refuses to save him from shrinking into oblivion, stating coldly her enlarging ray has only two shots left, and both are needed to help her cousin and Kandor.
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.
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.
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. This was near the end of Marvel's G1 run, where Ratchet was mostly played straight as a medic and counselor. When pushed, though, he became a Guile Hero who handed Megatron his ass. Twice.
In The Transformers: More than Meets the Eye, Tailgate is a gentle, friendly 'bot possessed of a childlike innocence. He also, in the second season, gains unique abilities that give him a truly ludicrous degree of Super-Strength. At one point he deals with a Serial Killer on board by picking up a shuttle dozens of times his size and throwing it.
The Ultimates: Nerd Hulk doesn't have any of the original Hulk's killer instinct and lack of self-preservation, but if you get him sufficiently riled up... he is still a Hulk.
Ultimate X Men: Colossus is usually a nice guy, but after weeks of being tortured by Weapon X, and having the chance to take revenge... he won't hesitate.
THE perfect example: Ozymandias from Watchmen. A genial and polite man who's made a friend/acquaintance out of most of the characters in the story. Turns out to be an evil chessmaster.
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 and especially if you hurt his loved ones. Or if you're a thug however loosely affiliated with the criminal group that hurt one of his loved ones.
Avengers long-time powerhouse Wonder Man has Sentry-level strength and speed, never tires, never sleeps, and is one of the most incredibly tough to hurt characters in Marvel. Why isn't he higher profile? He's nice. Too nice, in fact, just very much a regular Joe — that can bench press the Chrysler building. But insult the Scarlet Witch in his presence and he'll punch you completely through the Xavier school and partway through the Mansion itself. Just ask Wolverine.
Wonder Woman is one of the kindest heroes in the DC Universe, rivaling her good friend Superman in the niceness department. She is an ambassador of peace for her nation, and is literally a flawless goddess with no temptations or impurities (she regularly uses the Lasso of Truth on herself to keep herself in check). After the 1980s she's also one of the few heroes who has zero issue with using lethal force when it's needed. She comes from a society of highly-trained immortal warriors, and is one of the few beings with enough strength and skill to go toe-to-toe with Superman, so if you give her no choice, she'll snap your neck and walk off to solve the next crisis like it was Tuesday.
In Wonder Woman (Rebirth) sweet Big Fun Etta Candy has no problem shooting Sasha, her friend, in the head. Fortunately, it's not actually Sasha, but a robot duplicate.
Wonder Woman: The Once and Future Story: Etain is by far the sweetest and least assuming of the slaves. She's also the only slave to kill any of the slavers and their guards before the slave revolt and she gets away with it as she's quick, quiet and knows how to dispose of a body so it won't be found. She is quiet cheerfully enthused about killing the first guard while he tried to corner another slave too.
He 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.
Mrs. Jean Grey-Summers. Despite the fact that she has an occasional temper and she is most known for herepic rampage and tragic 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. For instance, she found out that Rich Bitch Emma Frost had manipulated Jean's deeply traumatised husband Scott (who had spent several months sharing a brain and body with Apocalypse) while serving as his therapist and convincing him that since it was telepathic, it didn't count. Emma taunted her. This was... unwise. Jean 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, intending to use the students as her raw materials, 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. Keep in mind she is not in Phoenix mode at the time:
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.
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?".
And while we're at it, let's not forget Jean Grey's 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 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. In the same comic he also single-handedly takes out a Sentinel that had been battering the whole team until then by taking off his visor and blasting it so hard the entire page went red.
Jubilee is a fun loving girl with a penchant for wisecracks and looking out for Wolverine. She's gone roller blading with Professor X when he had temporary use of his legs, once swapped Cyclops's ruby quartz glasses for a pair of Foster-Grants, and was devastated when Illyana died from the Legacy virus. She's also a walking nuclear bomb who holds back the bulk of her power for fear of killing someone. She has, in no particular order: blown up the Mandarin's mansion after thinking Wolverine was dead, beat up two hitmen who killed her parents, tricked Emplate into feeding on her first after poking at his family issues, ensuring that when he absorbed her power, he'd instantly lose control of them, and once blew up a godlike being by finally permitting herself to access the full measure of her power.
Of all the Young Avengers, shapeshifter Hulkling is probably the sweetest, most polite guy on the team. But... don't hurt his boyfriend Wiccan. They don't call him "Hulkling" just because he's green.
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. His mother being the Scarlet Witch.
He later helps convince a nearby Gelfling village to take up arms against a Garthim swarm. This when Gelflings were an artistically inclined, peaceful race who had believed Garthim to be indestructible.