The more I think about it, the more convinced I am that Kylo Ren was able to murder the rest of the Jedi students so effectively because he was the only one who had a lightsaber. I’m thinking Luke didn’t want to pass on that knowledge right away, but Snoke showed Ben/Kylo as a seduction tactic. The companion books refer to both Snoke and the design of the cross guard saber as “ancient”, so it wouldn’t be out of place for those instructions to come from him. I figure if Kylo learned from Luke, he’d have built a more conventional saber, and if Kylo didn’t learn from Luke, there’s no reason to think anyone else did. It would also justify his unimpressive performance against Rey, since in this scenario he wouldn’t need to have been a particularly skilled duelist to wipe out Luke’s academy or whatever he called it (Praxeum? 😉).

Of course, speaking of the Praxeum, wasn’t this basically what played out with Kyp Durron and Exar Kun?