@Quartz2208 - is the manslaughter by gross negligence not suggesting that he was negligent in getting her help once she had been battered and received no medical attention?
Is it that he was negligent in considering her welfare when he was engaging in "their" sex act?
I understand your point about the difficulty in a successful murder charge - but the charge just implies that he was neglectful - not reactive enough. Rather than proactively battering her to death.
That's how I view it and particularly why it feels such a shocking verdict.
EG - medical professionals, managers of businesses etc could be found guilty of the same crime when they were being neglectful, rather than attacking a victim.