Question: are all dark side users sith?
Answer: It is certainly possible to be a Dark Jedi or some other form of Dark Side Force-user and not be a Sith. ... A number of the Imperial Inquisitors were Dark Jedi, but not Sith. Likewise Asajj Ventress could be said to have been a Dark Jedi, but was never inducted into the Sith and was not trained in any of their sorcery.