I always figured it was anger and hate that turned Anakin dark, because that's what the dark side was all about in the OOT and that's what Vader and Palpy were trying to use in Luke to get him to go dark. I'd go so far as to say I suspect that may have been Lucas's original vision. Certainly the version we got in ROTS was a late development. And in an article on his site Michael Kaminski demonstrates how the heavy focus on trying to save Padme in Annie's reasons for going dark was a late development in the actual making of ROTS.