Kylo Ren is evil, but to paraphrase what Darth Vader said to Luke, “The Force is with you, young Solo, but you are not a Sith yet.”
He’s a moody teenager with little direction. He’s conflicted. He looks up to Darth Vader (his grandfather) as someone who had great ambitions but was “corrupted” by the light side. This is confirmed in this short interview with J.J. Abrams on the topic titled, “J.J. Abrams Explains Why Kylo Ren Turned to the Dark Side”:
Q: Kylo Ren idolizes Darth Vader, but Darth Vader turns good. So why does Kylo Ren idolize Darth Vader so much if he’s bad?
A: Kylo Ren idolizes Darth Vader, not Anakin Skywalker. He idolizes what Vader represents… What Vader was trying to do… The idea
that Vader didn’t succeed he was—if you look at it from Ren’s point of
view—he was seduced by the enemy. And failed because of that seduction
and so the idea is that Ren wants to complete the thing Vader started.
Q: And what is inside Ren that made him turn bad?
A: An incredible power. An incredible force and incredible potential. That was… Like with many young people misguided and
unclear. And the story for him is one of conflict… Not just external
conflict but internal conflict. And it’s what makes him what I think
is a rather interesting villain.
The hard part about being a Jedi is the temptation to slip into the evil encompassed in the dark side that would change who you fundamentally are. The hard part about being Sith is you have to—essentially—kill yourself to become something that is no longer connected to you; you become a monster and can never go back.
Generally—not just in the Star Wars universe—it is easier to be nice/light/good as a matter of basic existence. How many of us wake up each day to create acts of evil and chaos as easily as we breathe? Few to none and those people who do engage the n constant acts of evil are considered psychologically imbalanced because they do not fit into the rest of the world where the vast majority of us are simply good by default.
To accept a dark side of yourself—and your interactions with the world—you really need to make a strong effort. Yes, at times performing acts of evil seem like the quick, easy and even justified way to act. But most of us when confronted with such a choice just want to move on and get it over with. To force the constant conflict that evil demands is really a painful and strenuous thing.
In the case of “The Force,” the power the Force has amplifies this struggle on a seemingly biological level: You get more out of the Force the more you put into it. But since being good is a natural state, amplifying the good in you makes you better but it takes longer to achieve that strength. With the dark side of the Force, you get immediate gratification but it slowly kills you. To master the dark side you have to truly kill yourself in a way in which you will be consumed to the point you are definitely a part of something larger but lose a part of your inherent persona.
Jedis are Jedis and only amplify who they are since being good is a natural state; Jedi are truly “super human” in that respect. Sith need to become something else and can never go back. Any rational person who has all of their limbs dismembered wouldn’t even get to that point. Chop off one arm—or even a hand in Luke’s case—and most people will just “wake up” and realize they are slowly killing themselves and stop.