If Luke knows that Ren turned to the dark side, then he's a jerk for going into hiding. He should feel obligated to stop the man he trained from taking over the galaxy. Instead, he's just like, "Whoops, I gave an evil guy the power to take over the galaxy. Man, I'm in soooo much trouble. I'd better hide before my sister finds out."
The only viable reasons for Luke to go into exile, would be
1. If he needed to do it in order to defeat Kylo/Snoke. Like they were such a huge threat that Luke needed to focus on training himself to become even stronger with the force, and that kind of focus required complete isolation.
2. (and this option IMO would be the coolest) If Luke started to feel a strong pull to the dark side, and he worried that he could be turned just like his father was, and like Kylo was. And so we're faced with a Jedi who's not really good enough to beat Snoke, because he's so conflicted internally, but he still trains Rey while he deals with his dark temptation, and he tells her that she'll likely have to kill him when her training is complete because he's losing his will to resist the temptation.