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 Jerk Ass.
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 Yellow Jacket, eventually getting to the point he tried to cause a robot attack he could foil and look heroic. After hitting rock bottom though, he built up and became a Nice Guy again after some development.
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.
Due to her Trauma Conga Line, Teen Jean Grey brainwashes Warren into submission twice! And now in Battle of the Atom she manipulates Teen Scott by flattering him (after kissing Hank) to help her stay in the present, which will cause an Apocalyptic Future.
After Teen Scott leaves, Kitty Pryde 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.
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 Archie Comics' Sonic the Hedgehog, 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. Many fans were quite happy when the Reboot hit and he was back to being a generally nice guy again.