Maul dropped his affiliation to the Sith after his disappearance. And on here Siths are part of the Sith Empire faction by default. But you could have a former Sith doing w/e I'm pretty sure. (But you need to take into account dark side corruption if not redeemed.)
Hmm... I was thinking, what if a force-sensetive went corrupt because of his entire life being destroyed by Sith. Then he might want to kill sith, and still be a sith himself? Would that work?
Well. Sith is an organization. If you are trying to destroy the Sith organization as a whole and you happen to use the dark side of the force, then you really aren't a Sith. Simply a dark-side force (if so) user who happens to hate the Sith.