I can still entertain a notion that this was not, at least originally, a sex offender type of crime.
It's not totally unthinkable to me that BSL might have hit Mickey accidentally, panicked and grabbed her up because of fears of going back to prison, further alienating his gf's family, losing any ground he felt he'd made in attaining a "normal" life. (Not a good thing to do, of course, but still not the same as stalking a victim.)
I think he could have planned to hold her, alive, while he thought things out. But maybe she got a chance to attack him (because of course she would not be wanting to be held captive, whatever his motivation) and he killed her then -- or maybe she died of her original injuries. Or maybe she escaped from him at some point and he pursued, caught up, there was a struggle and he was injured then.
All the things he appears to have done to "cover" could still have seemed necessary steps to him. If he made a panicked decision to seize her in these circumstances, I think there would be no turning back for him later.