Okay, so let's say for a moment that during perfectly consensual rough sex, he'd killed her. That's still murder (or manslaughter) and he would be convicted easily.
Consensual rough sex shouldn't leave marks or lasting damage of any kind, if it does, you are doing it wrong, holy f***balls. It's about control, dominance and the illusion of danger, but it should always be 100% safe.
This is the reason that every single person you know who is into BDSM (and it's more than you think...) isn't walking around with bruises on their face constantly. If she does, in fact, have a skull fracture the fact that she requested rough sex doesn't matter, that's assault.
I'm not saying he's guilty, that's not for me to decide, but what has been released (from both sides) certainly looks bad.