Star Wars has a relatively even split between Sith who began as Jedi and Sith who did not, at least in terms of Star Wars ...

the Jedi of the prequel era have long strayed from their focus on the Force, losing their identity after the Sith went underground and becoming politically intertwined with a dying system.

"Once more the Sith will rule the galaxy, and we shall have... peace." More than an iconic line in Star Wars: Episode III - Revenge of the Sith, this sentence embodies what the Sith are all about ...