This is, in my opinion, a massiv thing that will come up in the next movie or the final of the trilogy. I stumbled across this by running my mouth in a conversation with some workmates today. They were unclear on what Rey's flash/force backs were all about and I started going and in mid sentence I realized "holy shit, Kylo actually killed the guy that was attacking Rey" (assuming she was seeing it from the perspective of her younger 5 year old self at the academy). Kylo has some serious issues with toe-ing the line between light and dark. My theory is that Rey was the "one step too far" that Kylo stopped at, which is why its a shock to him when he finally recognizes who she is in the movie.