In Star Wars 9, Kylo Ren already looks like he'll have a difficult time living up to his most famous quote: "Let the past die. Kill it if you have to." The new Supreme Leader of the First Order styled himself as a mentor to Rey in Star Wars: The Last Jedi and he urged her to join him in ruling the galaxy. Rey refused because she clearly saw that the former Ben Solo doesn't want redemption, although he may not be able to fully embrace the dark either.
Kylo Ren has certainly shown that he's willing to kill the past; he murdered his own father Han Solo in Star Wars: The Force Awakens and master Snoke in The Last Jedi. Kylo is the product of two warring philosophies of the Force: the light from his parents, and the dark from his grandfather and Snoke's teachings. This inner conflict that tormented him is what provoked Kylo to kill his father, thinking that that would finally turn him fully to the Dark Side. Ren's own ambition, desire for power, and hatred for Snoke then motivated him to kill the Supreme Leader. Ren also intended to kill his uncle and Jedi Master Luke Skywalker on Crait. But Kylo Ren struggles with his own (flawed) philosophy because killing the people who represent his past and letting the past die are two different matters entirely.
Despite his lofty claims, Kylo Ren hasn't been letting go of the past at all in the Star Wars sequel trilogy - he's still wrapping himself in it and that starts with his entire persona, which he modeled after his grandfather Darth Vader. In The Force Awakens, Kylo obviously styled himself after the Dark Lord of the Sith, complete with his own helmet and spoke to the burnt remains of Vader's helmet (which is rumored to reappear in Episode IX) stating he would "finish what he started". There's been a whole movie of growth since, yet it feels like he was never truly off that track, which spells doom for Kylo in Star Wars 9.
In Star Wars: The Last Jedi, Kylo Ren's turning on his past sees him assuming the title of Supreme Leader after killing Snoke, but he's still following through on Vader's ambition to conquer the galaxy. And a descendant of Anakin Skywalker ruling the galaxy isn't exactly "letting go of the past", it's merely fulfilling the darkest ambitions of the past's greatest villain. Even his proposition to Rey that she rule alongside him is just a twisted version of what Vader wanted with Luke.
Further, at the end of The Last Jedi, Kylo was also humiliated by the past: he was fooled by Luke's Force illusion in full view of Hux and the First Order and they ultimately failed to capture the leadership of the Resistance. True, the rebel band was decimated but the key members survived to continue the fight so it wasn't a total victory for the First Order - the tables are expected to turn in Star Wars 9. More importantly, Kylo's visible anger at Luke indicates he really is incapable of letting go of the past - and Skywalker's parting words, "See you around, kid", means Luke's Force Ghost likely won't let Kylo Ren's reign over the galaxy be a peaceful one.
By the time of Star Wars 9, that reversion is looking to be solidified. Not only is Vader's helmet back, but so is Kylo Ren's. It's also rumored the Knights of Ren will finally appear, again tying Ben Solo to his pre-Supreme Leader days. The very facets of Star Wars that created Kylo Ren are once again defining Ben Solo's path. As it currently stands - through a mixture of Rian Johnson's deconstruction and J.J. Abrams' nostalgia, Supreme Leader Kylo Ren is failing to let the past die - and that could well come back to haunt him in Star Wars 9.
- Star Wars 9 / Star Wars: The Rise of Skywalker (2019) release date: Dec 20, 2019