We knew going into Star Wars: The Force Awakens that Kylo Ren was a big Darth Vader fanboy, but the reasons why are not really elaborated upon in the movie. Of course, it is revealed that Vader is (SPOILER alert) his grandfather, but surely Han, Leia and Luke would have explained that Anakin Skywalker ultimately rejected the dark side and brought balance to the force - so why was Kylo (or, Ben) so enamoured with and taken in by his Sith history?
J.J. Abrams offers a pretty good explanation during an interview with IGN.
“Kylo Ren idolizes Darth Vader, not Anakin Skywalker. He idolizes what Vader represents and what Vader was trying to do. And the idea that Vader didn’t succeed, if you look at it from Ren’s point of view, he was seduced by the enemy and failed because of that seduction. So the idea is that Ren wants to complete the thing that Vader started.”
The director also touches on how the great power in Ben Solo was able to be manipulated and turned to the dark side.
“An incredible power, an incredible force, and incredible potential that was, like many young people, sort of misguided and unclear. And the story for him is one of conflict, not just internal conflict but external conflict. And it’s what makes him a rather interesting villain.”
I stated this earlier. Honestly, only LUKE known of Vader's redemption. If Ren is a sympathizer of the Empire and learning about his own parents were responsible or taken it down and his own uncle killing his grandfather pretty much helped push him.