Pick three OCs. How would they explain the origins of the Sith if asked?
Var'lin is not only a librarian with a cybernetically enhanced memory. He has been interested in the history of the dark side from a very young age. He's read every book the Jedi know on this topic, as well as those they don't want him to read. It's safe to assume he knows everything about the origins of the Sith, about the Great Schism and the Hundred-Year Darkness, about the first Lords of the Sith and everything else. He has built up a huge factual knowledge, but he would be reluctant to talk about any of it. If he knew he could trust the person he's talking to, he would love to discuss this topic at lenght. He's a bit of a nerd.
Like most of my pureblood characters Sarvis buys into the tale of how the arrival of the Dark Jedi elevated her ancestors. She doesn't see them as oppressors or colonizers, but as saviors and heroes of her people. To her, the Sith as they are today are a product of both her species' ancient powers and knowledge, and the strength of those Dark Jedi like Ajunta Pall. On the contrary, she sees the Jedi as oppressors who have tried to eradicate her people and her culture at numerous times in galactic history. A fairly standard imperial viewpoint, as she hasn't really been exposed to anyone who would challenge that point of view. (Her brother Sarian has actually started to question this point of view since he has been exposed to the light side, and he's had deep and meaningful conversations with the occasional Jedi).
Maks doesn't talk much, but she would probably explain the origin of the Sith like this:
"It was a mistake. Not killing all of them. Sending them away to the stars in the hope they would come to regret their path. We've damned billions of people, entire civilizations to a cruel fate worse than death. And all of that just because we were to weak to make the tough decision." She's always felt that the Jedi are in some way responsible for the evil of the Sith, and she has vowed to do all she can live up to that responsibility.