Who are the Knights of Ren? Earlier this summer, lead up pieces led me to believe we'd be getting some answers to this question than we got, for example:
"The movie explains the origins of the mask and where it’s from, ...” Abrams tells EW. “...'He is a character who came to the name Kylo Ren when he joined a group called the Knights of Ren,” Abrams says. But that’s as far as the writer-director will go. What are the Knights of Ren? You’re going to have to wait until Dec. 18 to find out.--Entertainment Weekly, Aug. 2015
Unfortunately, the movie came and went and those answers I still long for. The only tidbit we got was from Supreme Leader Snoke acknowledging Kylo as the "Master of the Knights of Ren". I'm sure there's a good reason Star Wars: The Force Awakens didn't delve into the history of the Knights of Ren, and since that portion wasn't addressed, I'm going to go ahead and take a wild crack at it myself.
Ever since their first mention, I've been hooked on the word "Knights". As far as the cinematic canon is concerned, to date, there's only been one other order of Knights in the Star Wars Universe, namely, the Jedi. There is an implication of valor associated with the word Knight, and as we've read about Kylo he doesn't see himself as evil per se. Unlike the Sith who craved power, all we know of Kylo and presumably the Knights of Ren, is that they are seeking to finish what Vader started (more on this later).
I believe that the Knights of Ren are none other than Jedi that had a philosophical split with the order early in their history. The Jedi believed themselves to be servants and protectors of those without a direct connection to the Force, where the Knights of Ren saw their abilities as a sign that they were to be the ones in charge over those without it.
Unlike the Jedi, the Knights of Ren saw a benefit in drawing from both the Dark and Lightside of the Force, something that would be sacrilegious to the Jedi (Probably why Snoke is so fascinated by Kylo who he sees as a Lightsider actively choosing the Dark, versus falling to it ala Anakin). After a fight for supremacy the Knights of Ren were defeated and headed off into the Unknown Regions where they were largely forgotten as time progressed in the galaxy.
When Kylo promises to finish what Vader started, what is he talking about? Clearly it was the subjugation and domination of the galaxy. I believe the Knights of Ren see themselves as the rightful rulers of the galaxy, chosen by the Force to use their power to instill order in the galaxy, with Force users rightfully at the top of the hierarchy. While Snoke does seem fearful of Luke, his intent isn't to destroy the Jedi, as he is more concerned with stopping the Resistance from finding and bringing him back. Snoke seems more than happy to let Luke continue his exile and is more focused on establishing the supremacy of the First Order and wiping out the New Republic.
But wouldn't this just make the Knights of Ren just like the Sith? I would argue no, because I believe the main distinction is that the Sith are solely devoted to the Darkside, whereas the Knights of Ren will be revealed to value both elements of the Force even if they wield the dark to enforce their rule. Where the Knights of Ren see value in the methods of Vader and the Sith's method, they ultimately saw the Sith as competition, otherwise why did Snoke watch from the rafters?
Adam Driver's quote regarding the Knights of Ren I think adds some credence to this theory:
"It was a group that existed before him, that he was a part of. Their place within everything is maybe more of a satellite group than I would say ... This is really tricky." -- Adam Driver, L.A. Times, Dec. 21, 2015.
Satellite group to whom? Unless we learn they were an ancient predecessor to the Sith, I believe we will be shown the answer to be none other than the Jedi themselves. A technically "new" group to the movie going audience who have allied themselves with the First Order, who run into the remnants of the Knights of Ren in the Unknown Regions, in order to capitalize on their philosophical rivals currently weakened state. This would also help us understand Luke's exile as being more than just a sulky Jedi bummed at his failure, and someone looking for answers at the origin site of the Jedi. Hopefully Episode VIII will shine some light on the Knights of Ren and their origins.