[Russ]

One possible idea that's been mentioned - when Ren says "I will finish what you started" he may have been referring to Anakin rather than Vader, suggesting he's on some sort of path where he feels he needs to 'go Dark' to complete his destiny, maybe to win Snoke's trust in order to defeat him. As the Light Side would never allow the death and evil Ren has caused it would be likely he'd eventually make a noble self-sacrifice at the end.