In my opinion, If RL was the murderer he would've been charged with double counts of murder by now, not probation violation for drinking and driving.
I have a theory that BG came across the girls right after they were dropped off. BG immediately became infatuated with ABBY and LIbby noticed this but kept it a secret (not telling ABBY) thinking nothing untoward would come of it. As they walked past him, BG stops to glance back at his prey walking deeper into the trail and deciding right then and there to bring his fantasy to fruition. In my opinion he deliberated with himself of how to carry out this crime. There was an extra person he had no use for, but to get to ABBY he had to subdue both of them or risk losing ABBY forever. BG, in my opinion, was angry that LIBBY was there. She was his only obstacle in his way. BG made his way back up the trail to carry out his devilish deed, but to his consternation, the girls were on that rickety bridge! But the plan was in motion and he wasnt going to allow no bridge to stop him, and besides they were still in his domain. At what point ABBY and LIBBY noticed him I dont know. My theory is BG never focused on LIbby thus giving her ample time to record him. In my opinion i believe BG would not of attacked LIBBY if she was there alone as she is not his ideal victim type. Ted Budy had his type, jeffrey Dhamer had his type, BG too has his preferences. This only my theory.
Another theory I have is BG took a memento from one of the victims he most preferred and it could've been anything from earrings, underwear, rings, ID cards, necklace, et cetera. I believe that none of these items were recovered at RL home thus excluding him from the crime. Unless RL dug hole and buried the girls belongings in a tiny chest.
Another poster on here had mentioned her theory of a duo or a pair of killers working in tandem with each other. Although rare it is not unheard of in the annals of crime history. If that is the case, one killer has dominant qualities and the other has submissive qualities. In my opinion if we're dealing with two devils in this case then BG is the more dominant personality of the duo as it was him who solely corralled the girls.