Question: is maul still a sith?
Answer: As Maul is neither, he cannot be Sith. ... Had he killed Sidious and Dooku then he and Savage would in-fact be the true Sith Lords, and Sidious/Dooku the rogues). By the time of Rebels, he's a former Sith/dark side force user (as evidenced by his own comment "Formerly Darth, now just Maul.")