Exactly, I agree with the above poster. The Sith cause imbalance, yes, but the Sith aren't the only thing on the Dark Side of the Force. In the Expanded Universe, for example, there were still Dark Jedi, whilst not Sith. While this isn't necessarily canon, it still supports my views.
Anyway, yes. "The Sith" is a belief of sorts, a way of looking at the Force, a thing to be practiced. You can be a Jedi and be evil, hence a Dark Jedi, and not follow the practitions of the Sith, see.