I have never seen Smallville either, so likewise, I cannot comment, but just how you said things like "early season" and "seeds of his future" it sounds like this was a gradual turn, which I believe is the way any turn has to be to be convincing, unless it is something like brainwashing or mind control. Anakin had three movies to turn to the darkside during, and he only does it during the last few minutes of the final film.
Luke's "turn" in ROTJ was great because it was emotional, with him giving into his anger then realizing his mistake, rather than a retarded confused "should I do it? No I shouldn't! Or should I? Errr, what have I done?! NOooooooo!... huh? Okay, I'll be your loyal slave from here on out. Oh, what's that? Go kill a bunch of kids? Alright I am on it. I'll be back in time for dinner master, remember, I like the red table cloth with the little black zigzag design. And don't forget the candles!
Padme had to die anyway, I have no idea why that wasn't used as a device to get him to turn.
I can't think of any other examples of reasonable turns off the top of my head.