Speculation:
So, assuming that Rey is Ren's sister (I'm sticking with that theory until the movies prove me otherwise), could she be the reason he turned to the dark side? He makes a few mentions to Solo not being a good father, that she sees him as a father-figure, and that she "didn't miss much".
Maybe he finds out that they separated them at birth for safety reasons and it is why he becomes enraged at his parents and turns away from the Jedi. maybe Snoke offered a way to find her?
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