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.