But that redemption was built up to for six movies, no Star Wars film has escaped that shadow. Of course there are villains who do not get redemption, like Tarkin or Palps or Dooku, but they are all acting basically of their own volition. Vader and to an extent Kylo are much more controlled and intentionally twisted by their masters into villains. You could argue that by the end of TLJ Kylo is now uncontrolled by his master, but we also see how destabilizing and weak this makes him. If he continues in this haunted, weakened vengeful state it is increasingly difficult to take him seriously as a villain, much less the ruler of the galaxy.
But what if he doesn’t stay in that weakened state? Trevorrow’s script set up a somewhat interesting character arc for Kylo. He gains more and more Force power, but in the process turns himself into a soulless machine. It reminds me of a classic tragedy, where the tragic character is destroyed by his excessive hubris. It could be a very emotional end for the character, if you don’t cop out at the last minute and have him redeemed anyway.