Hmm, I didn't get all those shades of gray from the OT
The OT didn't even talk about the Sith, because Lucas hadn't come up with most of it yet. He played around with various ideas about the Sith in the earliest drafts, with them being pirates at one point, so he always had something in the back of his mind.
The first time someone tried to do something with the Sith in the EU was when Zahn wanted to make an alien race called the Sith who were subservient to Vader, which would be why he was called the "Dark Lord of the Sith", but Lucas rejected it and Zahn turned it into the Noghri instead. Pretty much all dark side users were just treated as "Dark Jedi" throughout the 80s and early 90s.
The "Sith-as-aliens" idea popped up again in the 90s with the stories about Exar Kun and Ulic Qel-Droma, where it's explained that the term Sith originally belonged to a red-skinned humanoid species from Korriban who were conquered by ancient Dark Jedi expelled from the Order; the Dark Jedi then took on the title "Lords of the Sith"/"Sith Lords" and as they interbred and over time wiped out the Sith Purebloods, the term "Sith" simply came to be applied to the Jedi's arch-enemies in various permutations (the Sith Empire that stemmed from the original Dark Jedi, the Sith Empire led by Lord Vitiate that was an offshoot, the Sith Brotherhood led by Exar Kun, Malak and Revan's Sith Empire, the later Sith Empire under Darth Ruin, the even later Brotherhood of Darkness under Skere Kaan, etc.)
The old EU was really mixed up about this stuff, mostly because the writers did whatever they wanted without a story group like we have under the new canon. It does give a really interesting sense of history in depth though, since they keep splintering, like real world religions do.
George never cared about this stuff and his notes during the prequel trilogy indicate that the Sith were founded two thousand years prior to TPM by a Jedi who split from the Order and came to be known as Darth Ruin. Then they fought a bunch of wars with the Jedi, got wiped out, and Darth Bane reorganized it. That was the extent of his vision of the Sith at the time of TPM, and The Clone Wars expanded on it a bit, but the Darth Ruin part never got canonized since it never appeared outside of notes.
