I do too, I think Ren ends up being a good guy by the end of his story too. He's too obsessed with Vader/Anakin and one of his tag lines is "I'm immune to the light"
"If you end your training now - if you choose the quick and easy path as Vader did - you will become an agent of evil." Yoda's prophecy for Luke will come true...
I don't think Luke would go evil. Why would he? For what purpose? I think he did, however, fail in training new young jedi's in some way. Why would they stop with him? There is a huge gap IMO. Something we won't fully understand until the series is over.
Just a theory... If they go this direction, we already saw a preview in Jedi. When Vader threatens Leia Luke loses his shit and beats him down. Definitely a Dark Side preview, even if he held back at the end. What if more of his loved ones are threatened or killed. He could come to the conclusion that the Dark Side is the only way to protect them... The same conclusion reached by Anikan.
Luke will become a bearded Jedi Master and mentor to a young Jedi, just like Obi-Wan and Qui Gon. He will die in battle with a young Sith, just like Obi Wan and Qui Gon, but probably in the next movie.
