Played with concerning Hank Pym in the Marvel Universe. Initially presented as a Nice Guy scientist, he had a number of Jerkass moments when he became Ant-Man, particularly his tendency to dismiss and verbally put down his love interest The Wasp. However, he really took a level when he had a mental breakdown and became the anti-heroic Yellowjacket, eventually getting to the point he tried to cause a robot attack he could foil to look heroic. After hitting rock bottom, he built up and became a Nice Guy again after some development.
Near the end of his carer as Robin, Tim Drake's personality took a dark turn, even using Scarecrow's fear toxin against gangsters. The reason for this was largely due to losing his mom, dad, girlfriend, best friend, other best friend, stepmother, and adoptive father in separate instances and then having Dick, Stephanie and Cassie all betray his trust. Mostly Dick who refused to even listen to Tim's theory about what had really happened to Bruce and instead decided Tim was losing his mind, culminating in Dick giving Robin to Damian, who had made repeated attempts on Tim's life.
The original Eric Needham Black Spider was this after his Unexplained Recovery as during Identity Crisis, he was shown willing to hang out with the very villains he originally swore to take down.
Nearly every incarnation of The Hulk goes through periods where he goes from Woobie to Jerkass Woobie to just plain Jerkass. Then again, it's The Hulk. His different personalities have different levels of jerkass. The most notable one was his Grey Hulk phase, or Mr. Fixit as he called himself. This Hulk enjoyed beating people a bit too much and even accepted to work as an enforcer in Vegas so he could earn money for punching people all day. His newfound intelligence allowed him to be much crueler with his foes than the classic Green Hulk ever was. For example, when attacked by the Rock and Redeemer, he took advantage of the Rock's spinning spikes attack, smashing Redeemer against said spikes and effectively killing him.
Jason, the protagonist of Long Ago And Far Away was once the heroic "Child Knight" who saved the land of Elvenwood from the evil Witch-Queen Nexis. In the years that followed, he grew up into a 30-year-old Jerkass.
A New X-Men example with Hellion. While already a tremendous ass to begin with, he somehow still managed to take another level after his hands got blowed up by Bastion.
Kitty Pryde: After Teen Scott leaves in All-New X-Men'', she acts like a jerk to Modern Scott and says she doesn't want Scott and Teen Jean alone, since Teen Jean showed some attraction to Scott... even though she admits that Scott handled the situation well. Then, she berates Teen Angel for letting X-23 leave, even though there's nothing he could've done outside knocking her out, which he probably couldn't even have done. As she berates Teen Angel, she half admits that the reason for her shift in attitude, temporary as it is, is that she's trying to keep her students safe and feels like she's losing control.
Illyana Rasputin used to be a sweet, innocent kid until she was kidnapped by a demon. After spending one decade forced to live among demons and learn dark magic, she often acts as an abrasive asshole. In Avengers vs. X-Men, she started imprisoning captured Avengers by having demons feed on their souls until they were comatose.
After her husband's death, Alura In-Ze becomes harsher, more callous, more unreasonable and more unpleasant. She snaps at everyone and becomes emotionally abusive to her own daughter. In Who is Superwoman? she treats Kara as a reckless, undisciplined kid or an incompetent, useless soldier.
Kara: Lana, what's wrong with her? Why is she so horrible to me? I do what she asks. I try so hard to help her, and all I get is her anger. Her disapproval. She wasn't like this when dad was alive.
Power Girl during her diet soda processing problem Dork Age.
In Superman: Doomed, the Doomsday virus causes Superman to snap at friend and foe alike.
In Superman: Grounded, Dick Grayson, of all people, has taken one when he becomes the new Batman after Bruce Wayne's was lost in time. He tells Superman he shouldn't become close to the people he protects, otherwise he risks making them targets for his enemies. Remember, this is the same Dick Grayson who is known for being nicer and more socially-adept than Bruce.
In the new DC Rebirth continuity, Superman's father Jor-El can be described as this. Instead of being the benevolent, humble figure we are used to, he is a Well-Intentioned Extremist involved in much more shady, underhanded and outright dangerous activities. Jor-El was part of a cabal of six secret rulers from different planets who ruled their galaxy from the shadows and met in secret, and he would later create the humanoid behemoth Rogol Zaar to act as a mercenary for the Circle and carry out their dirty work in the galaxy. He becomes worse when Dr. Manhattan, the catalyst behind the previous New 52 universe who also set in a chain of events leading to the Rebirth universe, kidnapped him at the second Krypton was about to explode and placed him in the most war-torn city on Earth. After he saw the evils of some human men, he remarked how it was unbelievable for someone to be so wicked as to deny another human food. Later, Dr. Manhattan locks him in a room with video monitors where he witnesses man's inhumanity to man across generations: the Crusades, the Holocaust, the Ku Klux Klan, and other atrocities. This broke him and made him feel like humanity did not deserve his son's protection. He adopted the moniker of "Mr. Oz." and manipulated humanity's deepest emotions of hatred so they would turn against each other: a man firing at immigrant coworkers, a tribal leader firing on his own people and refugees. At the end, as punishment for his many sins and repeatedly violating the laws of nature and science, the Intergalactic Council returns him to his original place in the timestream, of Krypton's destruction, so he would perish along with the rest of the Kryptonians.
In his earlier appearances, Curt Connors of Spider-Man was a reasonably sympathetic character when he wasn't the Lizard, and even as the Lizard retained some protective instincts towards his family. He was retconned into being Evil All Along, and as of The Gauntlet he completely lost control of the Lizard persona and killed and ate his own son. After that, his more sympathetic characterization likely won't be coming back any time soon.
The Evil All Along ret-con was pretty much ret-conned itself after the story that introduced the idea, as no other writer has used it since then.
Natalie "Envy" Adams from Scott Pilgrim Volume 3 took a several year course in jerk-assery and passed with an A+++. In college, "Nat" used to be a nerd who had a lot of anime stuff in her dorm and kept to herself. She was Julie's roommate and she ended up meeting Scott and dating him for a nice while. Her, Scott, Stephen Stills, and two insignificant others (one being Young Neil's older sister Stephanie) started a band called "Kid Chameleon" that ended up becoming pretty popular, eventually cornered by record labels. Over time, she began becoming more confident and selling her old possession for cooler things while slowly becoming more of a 'bad girl,' even using the alias Envy. However, she also ended up changing the band without the consent of Scott or really anyone with the rest of the band against the changes. In fact, it's implied that Envy may have begun cheating on Scott and certainly saw less and less value in ther relationship. Eventually, an incident at the bar led to a drunk Scott and Envy creating a large argument that Scott didn't recall anything, but Envy announced that it was over, both the band and their relation, causing heartbreak for Scott and a good deal of insecurity. Byt the time we see her in the main line, she has become pretty successful though Scott has compliations talking with her while Wallace is pissed off for what she did to Scott.Now in a band with Todd Ingram, her now boyfriend, childhood friend and Ramona's third evil ex-boyfriend, and a drummer named Lynnette Guycott. In bitter irony, Envy discovered Todd himself wasn't as loyal as she thought and with Todd's defeat and Lynette's disappearance, she was left with little else but the clothes on her back (including an old hoodie from Scott). The two didn't make closure until the final book where Envy seemed to have found success going solo, albeit being with Gideon. With his end though, not sure what happens to her, but at least the closure helped repair the damage between the two.
In The Just #1, the older heroes of Earth-16 have no issue with re-enacting major battles for fun, regardless that some of their closest friends and allies actually died in those battles. For example, they have Red Amazo (the combination of Red Tornado and Amazo) participate in the recreation of the event that actually created him, but pretty much killed the Red Tornado in the process.
Age of Bronze: In A Thousand Ships, Paris starts out as a slightly arrogant but relatively decent and enthusiastic youth who is a bit overwhelmed by his new life. By Betrayal, Part One, after three years as a Prince of Troy and being indulged in his behavior by Priam, he has become very arrogant, rude, pushy, and refuses to accept he could be wrong.
In Sonic the Hedgehog (Archie Comics), King Acorn in the pre-Continuity Reboot universe spiraled into this direction after the restoration of his body, disbanding the more battle-hardened Freedom Fighters to install his Secret Service and trying to force Sally into marrying Antoine, but went full tilt after his body was damaged by Antoine's Evil Counterpart Patch.
Hot Rod after becoming Rodimus in IDW's run of The Transformers. In The Transformers: Autocracy, his main concern was protecting the poor Cybertronians who lived in the slums of Nyon. By The Transformers: More Than Meets the Eye, he's become an egotistical jerk who in love with the sound of his own voice and loves being in charge even though he's a lazy slacker who pretends to be asleep whenever someone approaches him with something that bores him.
Huey, Dewey and Louie. One story had them complaining to each other about Donald Duck's many flaws only to suddenly realize that no matter if Donald is a big success or not, he still takes care of them, cooks for them, makes sure they're clothed and happy, and on the whole uses a lot more money on them than he does on himself. Cue guilt and a resolve to start appreciating their uncle a little more in the future. Later stories have them be openly ashamed on him and even come off as selfishJerkassesnote In "Where There's Smoke", one of the three said he wasn't annoyed by Donald dropping foam on them (because Donald thinks that where there's smoke, there's fire), but because he would get the badge instead of them, and the comic portrays them in the right for their It's All About Me attitude, and they even get rewarded for it instead (they get their badge at the end), and in "Paperino e i Gamberi in Salmì", Scrooge's brother Gedeon gives Huey, Dewey and Louie a stand-in of the "Pulitzer" prize, but gives it to Donald to keep it for them until they grow up. Afterwards the three are shown running Donald out of town and saying "You're not our uncle... you're a medal snatcher!" just for getting the medal, which he didn't steal it from them., and it doesn't help that they stilltake the moral high ground against Donald and win.
The Life and Times of Scrooge McDuck shows Scrooge McDuck's decades long quest to become rich turned a nice, hopeful kid into a cynical misanthrope. As commented on by his sister Hortense in The Empire Builder From Calisota:
Hortense: Getting richer and richer, and meaner and ornerier! That's all you do.
Asterix: After spending the entire series as a joke character, Brutus suddenly becomes the Big Bad of Asterix and Son. He goes further than any villain previous and burns the village down. Not to mention, it's heavily implied he was trying to murder his baby brother in order to remain Caesar's only heir.
Battle: The Scholar becomes more obnoxious after Grogan's death, leading Charley to realise that Grogan might have been right about him thanks to becoming pally with Snell, who helps him with an application for officer training. When he returns to the front as a 2nd Lieutenant, he is outright snobbish towards Charley.
After Jenny Sparks died, The Authority as a whole did this: lethal force was no longer merely an option, but their only solution to dealing with their problems; they often acted unilaterally without regard for the consequences for their actions or the concern of others; and for a brief period, they took over the United States.
Nightcrawler started as a nice guy, as the mainstream version. Then he had an homophobic reaction when Colossus came out of the closet, and then towards all the X-Men.
Initially, Pyro helped other mutants and even joined the X-Men. Then came The Ultimates 3, where he was not only turned into a villain for no real reason, but suggested he and Mastermind rape Valkyrie.
While Dr. Destiny was always a villain who crossed paths with the Justice League of America, prior to the events of the first arc of The Sandman, he never engaged in the behavior he did in that story, which infamous including torturing the people in a diner for a day.
The Punisher: Detective Molly von Richthofen was a Jerk with a Heart of Gold at worst when she first appeared in "Welcome Back, Frank", being the only character to not completely treat Detective Soap horribly despite being annoyed by his whining. When she returned in the arc "The Resurrection of Ma Gnucci" in Punisher War Zone 8 years later, she's now an insanely jealous Psycho LesbianYandere who brutally attacks and beats anyone who even looks the wrong way at her bisexual partner. When she ends up getting suspended and her partner later breaks up with her (surprisingly, not for any of the unprovoked beatings she delivered), when she comes across a punk who's traded barbs with her in the past, she takes things out on him by shooting and killing his dog when it pees on the street.
The title character of Funky Winkerbean was something of a pessimist as an adult, but still mostly a nice guy. After Time Skip #2, he became a grumpy, avaricious Jerkass.