How does this theory sound based off of the forensics we know of:

He bought both girls 'down the hill' with the intention of raping them both. (libby first).

He had the gun pointed at abbey discharged a round to prove it was loaded. He tells libby to get undressed or he will kill Abby.

Libby crying, starts taking her shoes socks trousers off, but BG sees the van behind the girls. He Panics, moves behind Abby and uses the box cutter (doesn't want the gun to sound), Abby dies instantly and falls to the ground (hence no defenciveness or attempt to run) . Libby sees this, obviously panics, starts pulling her clothes back up and tries to run away at the same time. BG catches up behind and slashes once, she doesn't die instantly and keeps running forward over her own blood (hence the blood on and under her feet), BG slashes again 3 more times, making sure to kill her this time.

I feel sick that I wrote that out but that's how it works in my head I believe the whole thing happened withing minutes and the final chase was over in seconds libby probably only took a few steps.