The 'Lighter' Side Of The Force
one great fault in the theory: Vader has never been part of the hierarchy of the empire. He was the right hand of the emperor. he had the authority the give orders but he ruled with fear. I think he turned to the dark side in the moment he unsolicited killed the younglings, last he forcechoked the woman he loved and tried to save. if this isn't a big sign he turned to the dark side, i have no idea what else could be the best sign.but there is no mo question, he turned to the dark side the save Padme but failed really hard.
He turned to the Dark Side when he agreed to read the romantic dialogue in episode 2. That crappy script could have turned Santa Claus into Hannibal Lecter.