To clear this up:
"Dark Jedi, also known as "Fallen" Jedi, were Force-sensitives, frequently former Jedi, who chose to deny the light side of the Force and/or follow the dark side. Although "Dark Jedi" originally referred to a Jedi who had fallen to the dark side, it could also refer to uninitiated Force-sensitives who received no Jedi training but began their careers under another Dark Jedi. Others were simply dark-side users who did not follow the teachings of the Sith or other dark side organizations. In some cases, Dark Jedi also included "artificial" Force-sensitives who served the dark side, such as the Reborn and the Shadowtroopers."-(Wookiepiedia)
Edited to add more to quote.
Well, the sith really become what they are in swtor becuase banished jedi land on koriban, so there could be some confusion there. But I think I agree the dark jedi are just fallen jedi, and if you want to be really restrivtive, there are always just the true sith, instead of the order as a whole.