I’m sure we can all reel off zillions of examples where a character who is introduced as an antagonist, over the course of a franchise becomes one of the ‘good guys’. What examples are there of the opposite phenomenon?
Characters like Darth Vader who are good-becomes bad over the course of the in-universe time but were introduced to the public as antagonists don’t count.
Characters who are only “bad” due to an external force (mind control, having your soul ripped away etc.) don’t count.
Characters should have at least one complete movie/season to be entrenched in the “good” mould. Characters who are introduced in a movie/season and at the movie climax/season finale are revealed to be bad guys don’t count.
Warden Lopeman(Henry Fonda) in There Was A Crooked Man…(1970). Extremely straight arrow throughout the film, finally at the end of the movie, he is corrupted by greed, murders his captured escapee(Kirk Douglas) and grabs the hidden bag of loot.
Which has a rattlesnake inside.
Kinda weird that this trope doesn’t see more use acutally. As you say, the opposite case gets used a lot.
Sauske in Naruto more or less counts, though so far he’s spent most of his career as a bad guy killing other, worse, bad guys.
Jean Grey in the X-Men movies, though she sort of breaks your second rule in reverse. She was “mind controlled” into being good, then turned bad when it broke down.
With your exclusions, particularly the bit about being in for a whole season as good, I can’t think of a single character. It is interesting that it’s so easy to think of examples of the opposite.
I’m pretty sure it’s outside the framework of the OP, but I think what happened to Chris Walken’s character in The Deer Hunter deserves mention.
Also, those movies and shows where the “Jim Jones Syndrome” is at play, and the leader of some group of religious zealots goes off the deep end and does in his flock, could be squeezed into the idea.
That outfit (In Real Life) waiting on the comet and all drinking some quality Kool Aid in their sneakers, would be a good example after Jonestown and Waco and that sort of thing.
Would Travis Bickle (Taxi Driver) have ever been seen as “a good guy”? If so, his transformation goes from “good” to bad to “good” in the course of things.
Captain C.G. Culpepper (Spencer Tracy) in It’s A Mad, Mad, Mad, Mad World(1963) chases the various groups that are chasing after the money, then impulsively decides to grab the money himself and disappear.
For most of the Mission: Impossible TV series, Jim Phelps was not just a good guy, he was their leader. But in the abomination that was the 1996 film, he was revealed to be a murderous traitor. (Yeah, I know, different actor, different time setting, but clearly meant to be identified with the original character.)
I didn’t see it, but from what I understand Tony Almeida in 24 meets the criteria.
You don’t mention if it is alright if said character returns to good status, if not then the field is a lot wider with characters like Willow in Buffy.
IIRC, John Astin played the upstanding sheriff of Angela Lansbury’s hometown for several seasons of Murder She Wrote. Eventually, though, he turned out to be the murderer in one of the episodes.
How about Gaeta? He may not have turned evil, per se, but he ended up doing some very evil things. Of course, BSG morality is never black and white, but he did die a rather dark shade of grey; especially for someone who started out as one of the most decent characters on the show.
Londo Molari in Babylon 5 went from decent though buffoonish into a man reveling in genocide (though he reformed later on).
Though I didn’t follow the show, Lex Luthor in Smallville starts out as Clark Kent’s best friend and is far from evil. In this case, though, you know how he’ll end up.
Michael Yates on Doctor Who was a UNIT officer who eventually went bad and was kicked out.
And, of course, there’s Anakin Skywalker in the Star Wars I, II, and III.