Ever since TFA told us that Han and Leia's son went to the Dark Side after years of training with Luke, SW fandom has been assuming that Luke was running some sort of Jedi Academy in which Ben was only one of several if not numerous students. But there's actually nothing in what Han says on-screen, or elsewhere in the script, to say so. Here's the actual quote:
"He was training a new generation of Jedi. One boy, an apprentice, turned against him, destroyed it all. Luke felt responsible... He walked away from everything."
"A new generation" can mean a group of young people of the same or similar age, but it could also mean an individual who represents the next generation -- i.e. that Luke was training Ben, his nephew, to become a Jedi like himself. In fact, that's what Claudia Gray's recently published, canon-compliant novel Bloodline (which takes place 6-7 years before the events of the film) implies -- because in it we find that far from being able to visit Ben at some established institution of learning surrounded by his fellow students and apprentices, Leia often has difficulty contacting her son or even knowing where he is because he's off travelling to unknown parts of the galaxy with Luke. Which seems like a strange thing to do if either or both of them were responsible for training a group of prospective Jedi.
Consider also this interesting remark by current Star Wars canon keeper Pablo Hidalgo*, when somebody asked him how long ago the attack on Luke's "academy" had been:
@Skysawyer Let's not call it a 'Jedi Academy.' But it wasn't that long ago. Timeline will be told through stories, not tweets. :)— Pablo Hidalgo (@pablohidalgo) December 22, 2015
Which bears out the idea that Luke never had such an academy (even if he'd perhaps hoped to build one). And we've already had it confirmed, again by Hidalgo, that Rey's vision of Kylo and the Knights of Ren on what appears to be a rainy battlefield is not a flashback to the destruction of Luke's academy, but something else entirely. (I have a theory that it's actually a glimpse of the future, but my reason for believing that is based on a shooting spoiler for Ep. VIII so I won't get into that here.)
Anyway, my point is that if Ben WAS the "new generation of Jedi" to which Han was referring, then all he had to do to "destroy" all of Luke's dreams was to turn against him and join the First Order. If Luke had been unable to find other Force-sensitives who were able or willing to train with him, and he couldn't keep Ben from turning to the Dark Side, it makes sense that Luke would blame himself, conclude that he was not ready or able to train anyone else, and retreat to the first Jedi temple in search of wisdom and perhaps a measure of peace. It would also explain why Luke looks so grieved when Rey offers him the lightsaber at the end of TFA -- because he knows she's strong with the Force, he knows she needs someone to train her, yet his bitter experience with Ben makes him fear that he won't be able to keep Rey from falling to the Dark either.
None of this is meant to negate what TFA clearly shows us, that Kylo Ren is a murderer (indeed a mass murderer, since he gave the order to massacre an entire village). He is guilty of great evil, whether that includes slaughtering a bunch of hypothetical fellow Jedi apprentices or not. But it's an interesting thought, at least to me, that his title of "Jedi Killer" (which is never mentioned in the film or the script, only in supplemental material) might well be more rumor than actual fact.
* Yes, I know his Twitter has kittens all over it and claims to be a parody account. That's a recent change, and an in-joke for those who've been following him through all the madness since TFA -- but it actually is Pablo Hidalgo.