It's difficult to entirely hate the guy, even now, because Finn remembers. Superior officers having sex with subordinates wasn't unknown, but Ren was better about it than most, having asked outright for consent and made it clear that, despite his reputation, he wouldn't do anything if Finn refused. And...well, sometimes Finn remembers rare nights in Ren's quarters, with a clingy, touch-starved Sith latching onto him like a limpet and not letting go, the odd moments of vulnerability despite Ren's ruthlessness.
Bonus points for:
- Rey picking up on a few of those moments from Ren's side when she's in his head.
- Ren talking to Finn about his doubts because, after all, he is just another stormtrooper and if it's Kylo Ren's word against his Ren will be believed, and he just needs someone to vent to sometimes.