Lex in Smallville.
Morgana in Merlin.
Mordred in Merlin.
Bill in True Blood.
I've yet to see a good, organic transition of a good guy to a villain. The journey should make sense and not seem to come out of nowhere. Maybe I need to watch more TV.
Yet, I think I've watched quite a few of credible transitions of villains to good guys. Weird.
I'm starting on GoT next. See what's the fuss all about.