It all depends on how you define what is "Sith" and what is "Jedi". On the one hand, they're both quasi-religious orders, so a person must be initiated to be considered part of the respected orders and either choose to leave or be formally expelled to be considered to be a member no longer. On the other hand, those terms are often used as short hand for adepts of the dark and light sides respectively.
So yes, it's possible for a follower of the Sith Order to reject the dark side and seek the light while still *technically* being a Sith, just as a Jedi can fall to the dark side while still *technically* being a Jedi. But that's all it is, a technicality. An "evil" Jedi is a Jedi no more in his or her heart...but that doesn't automatically make them Sith either, just as a light side Sith by definition cannot be a true Sith at their core. But again, that doesn't make them Jedi.
It's also worth bearing in mind that not all light side force users are automatically Jedi, just as not all dark side adepts are actual Sith. Asajj Ventress was not a Sith, despite what she liked to claim. The Krath were not Sith, they just used Sith artefacts and knowledge for their own ends and were lead on by a Sith ghost. There's also a whole grey area of force users (including the original Je'daii) that do not hold exclusively to one of the other but strive for an internal balance of the two.
That said, stories centred around the Sith would give plenty of potential for good story telling and strong characters. You don't have to like what a person is doing to care about what happens to them. It's all about the execution. Just look at 'Breaking Bad'. At his core, Walter White was a nasty piece of work, but it's still a great show thanks mostly to that character.