My gut tells me that the trailer was a complete misdirection play by Johnson. Here's my very top of the trees theory:
Rey is turned to the darkside by Snoke
Kylo makes a decision at a very pivotal moment that turns him back to the light
You don't come back to the light after killing your pops
Anakin found the "light" at the end after killing children, his best friend and countless others.
I'd argue he died knowing he was a tragic figure but tried to give something to Luke. Not sure how much redemption like that is even available for Ben
How about this for a spoiler? The Carrie Fisher scenes might make wanna cry. no: