John Locke from Lost (although we ultimately learn that bad Locke is not really Locke at all)
In Anime there’s Code Geass where the main character Lelouch who starts out wanting to make a better world and ends up turning into the very thing he hated.
Sisko??? Don’t you mean Gul Dukat?
I think he’s meaning the episode In The Pale Moonlight
Sick of the losses the Federation is taking in the war, Sisko enlists Garak’s help in getting the Romulans to join the Federation against the Dominion. Sisko soon learns that in order to save the Federation, he must violate the values for which it stands.
Michael Mancini in Melrose Place was a good guy in the first season and went bad after.
Meh, hardly an instance of a character turning into a “bad guy” in the sense of the OP. If we included every show where a character makes a morally ambiguous decision for the “greater good”, it would be a pretty long list.
I actually thought of Gul Dukat, who starts bad, turns good, then ends up really bad. But he was a reoccuring character, not one of the “main good guys”.
Can we make an exception to the “whole movie/season” rule if the character only exists in a single movie?
Oh, yeah, Gaeta counts for sure. He tries to take over and get everyone(pretty much) killed.
Oh, yeah. Too bad he was far from a main good guy in the first Burton Batman movie.
How can this count? He’s en entirely different character.
And, of course, TVTropes has a page: Face Heel Turn.
Ash is a good buy in Evil Dead 1. But in Evil Dead 2 (or is it Evil Dead 3? Or both?) he gets split into a good guy AND a bad guy.
There’s also Angel. He turned bad on two different series.
Yes. Probably the biggest example, best fitting this thread, would be when Hulk Hogan turned heel (bad guy) at Bash at the Beach to join the Outsiders and form the nWo.
But it wasn’t Angel turning bad, it was him losing his soul again and turning into a vampire. Angel himself wasn’t bad. (For a while in ATS it seemed like he was, but he wasn’t).
Well, on ATS they did play it off like a split personality but on BTVS it seemed like they were going more for “here’s the same guy, just now he’s evil”. They didn’t even use the name Angelus to distinguish him on BTVS, they only called him that in flashbacks and stuff, not after he turned bad.
I always figured that was because on Buffy, we were seeing it from the humans’ POV. On Angel, it was more Angel’s POV.
Or maybe it was inconsistent writing. Your choice.