I don't see how Sidious manipulating Kylo explains away that much (heck, he did the same to Anakin and we're perfectly happy to let him take responsibility for his fall). We already knew that Snoke was working on him, so it's so much new info as a clarification of old info. Besides, at the end of the day, Kylo wasn't raised in the dark side, like, say Maul; he had all the options to make the right choice and still failed himself. While I think that Kylo is relatable in terms of not being pure evil and having understandable motivations for why he went to the dark side, I find the "devil made him do it" argument to not work very well and undermine the attempted redemption story (if he wasn't "really" responsible, then his repentance looses all its weight).