Actually the only charecter we have ever seen turn away from the darkside in the movies was vader,and that only with the help/aide/intervention of his son.
I truly dislike the change sides feast that is KOTOR,on the last count there where handfull people changing sides,most of them after a single conversation with the player(player being lightside in this game),so is the darkside really so seductive and is there really a chance of it to "forever dominate your destiny",or is it just a mood thing..."player wakes up and feals grumpy" *oh i