Lord Scourge went Republic and he's a sith pureblood. There is also a part in the Sith Inquisitor story that a sith pureblood on Tatooine talks about how the empire is wrong in fighting genocide with genocide and doesnt agree with the empire at all. Many sith purebloods have different views from the empire and those IMO are true sith because they fight for what they want and not what the empire wants. They fight for themselves and their own purpose and not an empire thats whole purpose is to serve an emperor whos only purpose is to server himself.