Ben/Kylo didn't really willingly go to the dark side, rather he was manipulated and seduced by Hux because having the son of the leader of the resistance and the nephew of the Jedi Knight was too good an opportunity to miss. So Ben/Kylo ended up in a relationship with Hux, only for it to turn horribly abusive (can include non con if author wants) and controlling with Ben being punished for any 'mistakes' (mistakes as trivial as author likes)
While interrogating Rey,she discovers just how badly Ben suffers because of Hux and even though she feels conflicted she eventually convinces Ben to escape with her and return home.
When they arrive at the resistance Ben ends up passing out due to old or new injuries and a medical droid reveals the extent of the damage to Han and Leia.

Bonus Points
- Conflicted Rey (unsure because the damaged Ben she sees does not fit her ideas of Kylo)
- Cousin/sibling relationship with Rey
- Angsty, self blaming Leia (and Han) but maybe Leia felt some pain through the force occasionally but blocked it out and now feels guilty
- Angsty, self blaming Luke- Ben was his student, he feels guilty he couldn't protect him
- Angry Chewbacca and even angrier/hurt Han that he couldn't protect his son.
- Guilty/Hurting/Confused Ben
- Ben waking up with his family beside him (maybe thinking it's a dream and having to be convinced it's real)
Lots of Skywalker/Solo family feels.

Here's hoping someone takes this prompt :)