Alright, saw it a second time. I have a theory now.
Ben would have been one of the first of the new Jedi. Wookieepedia just says he was born after Endor, so lets assume he was born fairly quickly after. He would have been in the first class of Jedi to be trained by Luke. We know this because Luke was still the only Jedi, so something had to have happened (ie. Kylo Ren).
Rey was born 15 years after Yavin, likely making her, lets say, 6-10 years younger than Ben. If she was going to arrive, or had been scouted by someone to be a Jedi, Ben would have known. And with her natural ability, he would have realized that he wasnt the strongest or the best, and he wouldnt stack up against her, let alone someone like Anakin. So, he went the quick way to the Dark side, killing everyone.
Except one person escapes. Rey. With the help of the old man on Jakku, who is clearly an important figure if he knows what he knew, especially Luke's position. He would be the Obi-wan figure, keeping watch from the distance.
This is the best Ive got so far.