We know she was on her back with the bra on for at least a short while, as the aspirated blood droplets show.
So if she was on her back why try and pull the bra off from the front? Why not just slip the knife under the centre of the bra and slice through where it will then just fall open?
If it is cut from the back, or pulled apart from the back, as seems reasonable to assume given the above, he had to turn her over to do it, or perhaps she managed to turn herself somehow in a last effort to break free.
No, it's certainly not simple.
Perhaps he no longer needed to restrain her if she was laying on her back at this point. But how to explain the bra being cut or tugged apart from the back? It would have been the path of least resistance to use his knife to cut it open from the front.
How common is necrophilia I wonder? I think it takes a particularly depraved individual to be still thinking about sex while his victim is gushing blood, making who knows what kinds of sounds or faces, and or perhaps just expired.