Yes, spoilers for The Last Jedi. You haven't seen it yet?
Not that I intend to come around and bash Luke (he's just a person, after all), but isn't he a bit hypocritical here? He looks at Rey with wide, fearful eyes and says in shock, "You went straight to the dark." How scary for him! First Ben and now Rey, too! Poor Luke with all these evil apprentices.
Think about it, though. What was one of the first things that Luke did when he was training with Yoda? He felt cold (which is a sign of the dark side) and he felt called to go into the cave in the tree that festered with the dark side. When Yoda advised him to leave his weapons, Luke ignored him and went and had a vision of violence and darkness. He dreamed he fought Darth Vader and then saw himself in Vader. All very dark. But was Luke overcome by darkness, or did Yoda say, oh, my, you're so deep into the dark side, boy? No, no, no. Luke just had to be aware of the dark side. He went, he saw, he left, he moved on. A couple decades or so later, Luke needed to let Ben and then Rey do the same.
He looks at Rey with such fear. Sure, he has bad memories of Kylo, but doesn't he treat Rey a little unfairly? She was just aware of the darkness under the island in the same way that Luke was aware of the darkness in Dagobah. Rey even went into that dark place and had her own vision--and it wasn't even a very dark vision. She just saw herself, a visual image reflecting the very fact that it is her own choices that shape her identity. For all that Luke pretty much washed his hands of guiding Rey from the dark side (though honestly all he needed to tell her was what the Force was and what what light and dark mean when it comes to the Force), Rey handled herself pretty well.
And Ben? Luke's reaction to Rey makes me wonder how much Luke also overreacted with Ben. No, I'm not saying that he completely made up Ben's infatuation with or weakness toward the dark side. But Luke did make up the fact that Ben was automatically aligned with the dark side. He says now that such "raw power" didn't scare him enough then, but I think the opposite is true. I think Luke was so scared by Ben's potential to be a second Vader that he withdrew himself and he forgot that it was the very connection between individuals, specifically between family members, that brought Anakin back from Vader. Rey says it herself: Luke's mistake was acting like Ben's choice was already made. Did Luke create Kylo Ren? No, everyone makes their own choices--but Luke did label Ben as something dark and evil. And Ben, already conflicted, believed him.
The first ill that Luke did Ben was to pull out a deadly weapon against him. The second ill was to tell him the lie that he was born of the dark side, that he was darkness who could only rise to be darkness. That is the lineage that Ben received. He was already an adult when he learned that Darth Vader was his grandfather (and that his parents and his uncle knew all along)--and then right after his uncle tries to kill him. So he embraces the darkness, the only thing he has left.
But isn't part of it true? We are all born into a certain darkness. In our galaxy, we call it sin. In the Star Wars galaxy, it's the dark side. Everyone has that legacy and that capability. The important thing is to choose the light over the dark. Kylo's still stuck thinking that the darkness is all he deserves. Leia gave up on him. Luke gave up on him. Snoke was just using him. Now even Rey has closed the door on him. So he's left holding the locks on his own chains, ignoring the key because it must just be a phantom vision, it can't be something real.
Ah, Luke, you didn't put the darkness in your apprentice--but you might have forgotten to tell him about the light of hope that extinguishes darkness.