Kylo Ren committed numerous horrible acts in Star Wars: The Force Awakens, chief among them stabbing his own father. You'd think that alone would be enough proof that he's fully crossed over to the dark side. However, according to Adam Driver, when we reunite with Kylo later this year in Star Wars: The Last Jedi, Kylo may be more negatively affected than expected over what he did in the 2015 Star Wars film. Driver said:
I feel like almost everyone is in that rehabilitation state. You know, I don't think that patricide is all that it's cracked up to be. Maybe that's where Kylo Ren is starting from. His external scar is probably as much an internal one.
During Vanity Fair's extensive coverage of Star Wars: The Last Jedi, Adam Driver noted how Finn, one of Kylo Ren's many enemies, will still be recovering from the injuries he received from Kylo Ren in The Force Awakens. That's why he's wearing that bacta suit in the trailer that came out last month. But Finn isn't the only one who will be rehabilitating when The Last Jedi picks up, as Kylo Ren didn't walk away from Starkiller Base unscathed. Not only is he scarred from his lightsaber battle with Rey, but as Driver said in the comment above, he might have lingering regret about killing Han Solo, thus churning up some inner turmoil.
Driver has previously said that Kylo Ren will exhibit more "humanity" in Star Wars: The Last Jedi, subsequently adding more depth to the character beyond the general traits and emotions we saw in The Force Awakens. Since Kylo Ren, formerly known as Ben Solo, isn't an embodiment of pure evil like Emperor Palpatine or Supreme Leader Snoke, having Kylo experience some second thoughts about what he did isn't entirely surprising. Whether or not those thoughts will eventually push him to the light or if he'll smother them and completely embrace the dark side of the Force remains to be seen.
There's been debate among Star Wars fans about whether or not Kylo Ren can be redeemed for killing Han Solo and initiating the elimination of Luke Skywalker's Jedi Order. Keep in mind, though, that Anakin Skywalker slaughtered younglings in Revenge of the Sith, and he still became a shiny Force Ghost after death. Whose to say that Kylo couldn't walk down a similar path? In any case, official details about Kylo Ren's role in The Last Jedi are being kept under wraps, as is the norm with Lucasfilm projects. Unofficially, it's been reported that he'll be wearing a Darth Vader-style cape, pilot a special TIE Fighter and travel to some familiar locations that his Sith Lord grandfather visited back in the day.
Star Wars: The Last Jedi will be released in this galaxy's theaters on December 15.