Well officially, a Dark Jedi is a Jedi that turns to the darkside and a Sith is some one adept in the ways of the darkside via Sith teachings. ie all dark jedi use to be jedi all sith did not. I don't really see much of a difference between the two theories. On one hand the force has a dark side that tempts you into becoming evil for the sake of power. On the other hand there is just the force but you are tempted by the power of it to use in evil ways. Either way you are succoming to a temptaion and choosing to be evil. Either way works for me. One thing about the christian view of it is that God (ie the light side) Created satan who turned evil by succoming to the temptation of power. But again either way works for me.