Kylo Ren may not have been a Sith, but in a delicious irony, he followed the Rule of Two better than Darth Vader. Ben Solo, the Chosen One’s grandson, grew raised knowing nothing about his origins. He only discovered the truth alongside the rest of the galaxy, when the fact that his mother was Darth Vader’s daughter became widely known. This appears to have shook Ben to the core, and he has become completely infatuated with his grandfather.
By the time of Star Wars: The Force Awakens, Ben – now known as Kylo Ren and engrossed in the dark side – had built a shrine to Darth Vader in his quarters. He maintained Darth Vader’s mask and attempted to communicate with the dark side through it. He promised to complete his grandfather’s mission, although it’s unclear what he thought it was. Whatever the reality is, Kylo Ren never became the towering icon of death incarnate that Darth Vader was. But there was one way he outperformed him.
Kylo Ren was presented in The Force Awakens as a terrifyingly powerful dark side warrior, although he was not a Sith. The Baneite Sith line terminated with Palpatine’s apparent death in Return of the Jedi, when the revived Darth Sidious refused to take on another pupil. Kylo Ren may have wielded a red lightsaber and studied Sith philosophy, but he was never a Sith.
From an out-of-universe standpoint, it makes obvious that Star Wars would not wish to bring the Sith back. This would have lessened Darth Vader’s sacrifice and victory over Palpatine. In-universe, Palpatine had always had a subtle influence on Kylo Ren, but Vader’s final act of treachery meant that the Emperor no longer wanted to observe the Rule of Two. Palpatine saw himself as the pinnacle of the Sith; a new student would merely be another potential traitor, as the Rule of Two required that every apprentice attempt to assassinate their master.
Kylo Ren’s betrayal of Snoke followed the Rule Of Two perfectly.
Palpatine had manipulated Kylo Ren through proxies, including Supreme Leader Snoke, a formidable dark side entity who appeared to have never realized he was a puppet. Snoke was Kylo Ren’s master, eking out knowledge of the dark side while continuously keeping an eye on his apprentice’s power level. Snoke and Kylo Ren may not have been Sith, but they were almost exactly following the Sith Rule of Two.
Snoke attempted to persuade Kylo Ren to kill Rey, and everything came to a head in the most dramatic and Sithlike way possible. Instead, Kylo Ren skillfully concealed his intentions from his master, manipulating circumstances such that his lightsaber sliced Snoke in two, killing him in a surprise blow. With that, Kylo Ren succeeded Snoke as Supreme Leader of the First Order. Ben Solo may not have been a Sith, but this was the most Sith-like act he ever done, entirely in line with Baneite traditions.
At the very least, Kylo Ren performed better than Darth Vader.
According to Star Wars: The Force Awakens, Kylo Ren believed he was carrying out a perverted version of Darth Vader’s mission. It is only fitting, then, that he did what Darth Vader never did as a Sith Lord: he assassinated his master and assumed his place. Darth Vader did betray Palpatine, but it wasn’t because of the dark side or the Rule of Two; his actions were motivated only by love for his son. Kylo Ren, on the other hand, sought Snoke’s position, despite his desire to share his kingship with Rey.
In general, despite his struggle with the dark side and study of Sith lore, Kylo Ren was never as dark as Darth Vader. He was always too aware of the light side’s call, having been imprisoned by the dark side through events orchestrated by Palpatine rather than freely choosing it, hence Kylo Ren was never truly lost. But in this one area, he outperformed Darth Vader and (briefly) reaped the benefits.