The solution now appears obvious: Kylo Ren must be a force of good. Is there evidence for this? Yes, in fact. He is secretly drawn to the light, and all characters know how conflicted he appears. It takes only a little more exposition to establish that he has secretly been attempting to destroy the Sith for his entire life, ever since he learned of Palpatine during his training with Luke. In order to get close to Snoke, Han had to die. In order to find Palpatine, Snoke had to be destroyed. Ben followed the ways of the Jedi, divesting himself of attachments to all those whom he was afraid to lose and forging a new identity for himself as Kylo Ren, a false identity taken merely to complete what Vader could not and destroy the Sith once and for all.
I really like this idea in theory, but I wonder if there’s too much going against it. If Kylo were good all along, why would he consider Finn’s actions that of a traitor? What is his obsession with having Anakin’s blue lightsaber anything but a sense of entitlement? Why was Luke so concerned in the darkness rising in Kylo that he contemplated killing him in his sleep after “Snoke had already turned his heart?” Why did Snoke say “as [Kylo] grew stronger, his equal in the light would rise?” (Then again, Snoke was wrong about a lot of things). Why did he double down on letting old things die & destroying the rebels while also snapping at Rey for still holding on and telling her she’s nothing? Why did he sacrifice total victory over the Resistance for a chance to kill what he thought was Luke other than out of vindictive pettiness? Why is he so adamant about killing “the last Jedi” & saying he’ll destroy Rey to get under Luke’s skin?
To bring an idea from the Duel of the Fates script, maybe Luke’s been communicating with Kylo between movies/at the beginning of Episode IX. Saying things like “I know what you’ve been trying to do, but you can’t do it this way.” You can also always redub his lines with Rey with phrases like “You asked me why I killed my father: to draw him out” or “You think you belong to the Light, Rey, but you don’t.”