The “Kylo was able to kill the rest of the Jedi so he should have also been able to easily defeat Rey” argument doesn’t hold any water because we have no idea what the status of the Jedi Order was when he took it down. It’s my belief that Luke might have been reluctant to teach his pupils how to use the Force violently, at least in the early part of their training; I even have doubts that he’d taught any of his students how to build a lightsaber at the time Kylo slaughtered everyone. Again, this is purely my conjecture, but there’s nothing in the film that contradicts this so it’s entirely possible. I think Luke was teaching his new Jedi a more peaceful, meditative approach to the Force than the whole warrior monk thing we saw in the old Jedi Order, and that Kylo learned his violent abilities from Snoke, who we know was already tempting him before he turned on the Jedi. So when he turns up to kill everyone, he’s the only guy with a lightsaber (which Snoke taught him to make, which explains why it’s such a different style than the one Luke built for himself), and the other students are totally unprepared for the confrontation. The way I think it went down, Kylo Ren was essentially a school shooter.

And the idea that he was able to do this simply because he was the only guy in the room with a lightsaber carries over to his status in the First Order. People complain that he’s been intimidating and effective up until the confrontation with Rey and that it’s out of character for him suddenly to lose his command of the situation at that point, but really all we’ve seen to demonstrate his power to that point is him bullying non-Force users. He’s a big fish in a small pond until Rey shows up, and he’s completely thrown off his game when he has to face someone else with access to the same kind of power he uses to lord over all the helpless normals.