I believe every jedi who falls to the drak side frist can be described as Dark Jedi, but then they can still turn to the light again, while a sith has passed beyond the point of no return and can no longer be saved(don't give me examples where this has not been the case). So while a dark jedi ccan still turn back, a sith fcuses so much on hate, fear and passion, that he is consumed fully by the drak side.
So, dark jedi: Not entirely evil.
Sith: pure evil