You can define pop culture pretty much any way you like.
It’s a staple of daytime soaps almost as old and ubiquitous as the magically aging children and the popular characters who rise from the dead when the actor gets a big new contract to come back: the bad guy- and not in the Fonzie ‘bad boy’ way but drug dealers/thugs/rapists- who prove super popular with the audience have a way of becoming good guys (usually after it’s revealed he had a bad child and is really the long lost grandson of the 45 year old matriarch on the program).
There’s also a revisionist trend in books, particularly those of Gregory Maguire (Wicked and others) where he re-writes a story from the viewpoint of the villain. This has been done by other authors with Fagin from Oliver Twist, Judas by several novelists (Frank Yerby and Taylor Caldwell among others) and in Jesus Christ Superstar, and there are other examples. Even I Claudius could be argued to do this somewhat (since the historical Claudius seems a lot bloodier and less naive than his fictional counterpart).
What are some examples of the reverse in literature/film/comics/TV/etc.: a character who is genuinely likeable and a good person transforming into a bad person? (I don’t count super villains who are nice in the first 4 minutes of a movie until a lab accident turns them into a levitating psychopath or whatever, but those who were characters before turning bad.)
A few I can think of:
On the current season of Ugly Betty her most recent boyfriend, Matt, seen as a nice guy at first, is gradually revealed to be a spoiled self consumed brat who’s being groomed to be next year’s villain.
On an episode of The New WKRP Tim Reid reprised the role of Gordon Sims/Venus Flytrap, who’s now a wealthy executive and pompous ass who doesn’t want anything to do with his former colleagues.
In the rumored but never filmed prequel trilogy to STAR WARS it was said that Lucas planned to turn Annakin from a hero into Darth Vader.
Roscoe Coltrane, while never actually the good guy on Dukes of Hazzard, was originally (in the first season, which was more comedy drama and more serious) a more sympathetic character who really didn’t like working for Boss but had lost his job and pension after many years as a straight arrow sheriff. By the time the show settled into its rut (second season or before) he was a cartoon stupid villain.
If the Bible counts as literature, King David goes from the giant slaying musician and hero to a middle aged lecher that in spite of having a harem has an adulterous fling that ultimately leads to having the husband killed to cover up an unplanned pregnancy with a quick marriage.
In mythology Theseus goes from minotaur slayer to insanely jealous of his son and his much younger second wife, while Jason goes from the hero of the Argos/Golden Fleece quest to a middle aged climber determined to better deal his wife Medea for Creon’s daughter.
What are some other examples?