Warning: massive spoilers ahead for Star Wars: The Last Jedi! Read forward at your own risk!
Now that Star Wars: The Last Jedi has finally debuted in theaters, it's clear that Kylo Ren has tumbled even further down the Dark Side rabbit hole. Yes, he did take down Supreme Leader Snoke in a moment that almost seemed heroic at first glance, but he then quickly backpedaled and embraced his newfound status as the leader of The First Order. This has led many to question if the former Ben Solo is beyond redemption, but The Last Jedi director Rian Johnson apparently seems to think that Kylo could definitely be brought back to the light, considering that Darth Vader also redeemed himself. In a recent interview, Johnson explained:
Are you kidding? Vader was worse than Kylo ever was, I think, and Vader got redeemed. Also, I should just for the record [say] that I'm not involved in the writing of the next movie. I'm an audience member in it, just like you, so when I talk about what's going to happen next it's in the context of, as a fan, what I'm thinking of.
When we look at the Dark Side turns for Darth Vader and Kylo Ren, there's a strong case to be made that Vader took things to a far more evil level. Mainly serving as an intergalactic analog for Heinrich Himmler, the father of Luke Skywalker personally destroyed the entire Jedi order, murdered countless people (including younglings in Revenge of the Sith), and took his place as Emperor Palpatine's right-hand man. Kylo Ren's turn to the Dark Side echoes many of those beats, but much of his arc has been defined by a stronger sense of uncertainty and emotional baggage. Plus, he never murdered any younglings (that we know of).
It's also arguably clear that Kylo Ren could return to the Light Side of the force because of the way in which he has modeled himself after Darth Vader. Rian Johnson doesn't know what the future holds for the character going into Star Wars: Episode IX, but as a member of the audience, he seems fairly confident that it's a possible path.
This possibility raises questions about how Kylo Ren could potentially be redeemed, and which characters in the current Star Wars franchise can save him. Elsewhere in his interview with IGN about the possibility of Ben Solo returning to the Light Side, Rian Johnson made sure to note that this responsibility doesn't (and never did) belong to Luke Skywalker. As Luke points out in film, he cannot personally save Kylo after trying to kill him years earlier; that burden will fall on someone else, with Rey serving as the most likely candidate at this stage.