Okay, MAJOR SPOILERS for Star Wars: The Force Awakens lie below, so you know the drill, leave if you haven't watched it or beware the spoilers you will find.
Now, we know that Kylo Ren turned from the light side and his tutelage under Luke Skywalker to that of the dark side and Supreme Leader Snoke. However, while it has been supposed up until this point that when Ren turned he also murdered the rest of Luke's pupils, which would indeed follow a similar path as Anakin Skywalker took when he started on the path of the dark side of the force...
It also seems strange. First of all, it seems unlikely that he would be powerful enough at that point to destroy them all by himself, and secondly it seems likely that another fate befell them all, possibly one worse than death.
You see, much like the original Star Wars film set up the trilogy without introducing many important aspects of the story (such as Yoda, the Emperor and more) Star Wars: The Force Awakens managed to introduce us to a wealth of new characters without showing us the whole of the danger behind the First Order. It focused on Kylo, and barely gave us a glimpse of Supreme Leader Snoke. But in addition to that, it gave us only a tiny glimpse in Rey's vision of the Knights of Ren.
Yes, these mysterious figures who are supposedly led by Kylo look dangerous, and while none of them have lightsabers in this shot, I feel it is quite likely that they are in fact all strong with the force, and once were the other students of Luke Skywalker.
Let me explain. You see the Knights of Ren are special for a reason. There must be a little more to them than some special training and a cool name and costume. The Knights in that flashback were supposedly finishing the destruction of Luke's Jedi Academy, and it's been rumoured that the Knights are a group Ren joined but did not create. If that was truly the case, how did he become the leader? Isn't it just possible that he brought them together? Maybe they were his friends at the Academy, and he slowly dragged them along with him into the dark side of the force. If they had gone to Snoke as a group with Ren leading them, he could have given them the power they needed to destroy the Academy.
I know that it seems unlikely, but don't you think Luke would have had the will to continue the fight against the First Order and Ren if one lone student had turned against him? It would make more sense that he would banish himself into exile if he truly considered himself a failure, after a whole group of his students turned to the dark side. And that's why Rey might just have a difficult time convincing him to train her in Star Wars: Episode VIII.
It may just be possible that the Knights of Ren all have something personal against Luke, and I imagine we will see them in the next film, and this mystery will be revealed.
Thanks for reading guys! If you like these theories about the latest news from Star Wars: The Force Awakens then click on the big blue follow button below, because there's plenty more where this came from!