I have posted something similar once before but I'll do it again, bc I think it is of importance to consider that the murders could be motivated by other causes than what is mostly discused:
My theory (one of them):
The killer's motive was to silence one of the girls, I believe Libby, bc she knew something about him that would ruin his life if it came out, and that Libby may have threatened to tell what she knew. Abby was just at the wrong place at the wrong time and the killer choose to kill her too bc she was wittness.
The killers MO was that of a lust killer, possibly sexual motivated, and that he choose that MO to make the murders look as something else than it actually was in order to misslead police.
How would the killer know that Libby was on the bridge?
If the killer was facebook friend and/or Instagram follower of Libby maybe Libby might have posted on FB (privat message to someone?) or snapchat on Instagram something about going to the bridge so he would know.
Or he was stalking Libby, waiting in a car close to her home and followed when Libby and Abby left Libby's home by car ?
Also, we know that there was several in Libby's close circles that knew ( not that I suspect any of her family members) that the girls was going to the bridge, maybe one them have mentioned something to someone, by phone, or social media like ; " -Ok, but I just have to bring my sister (or; my daughter etc) to/from the bridge then I have time to



" and like that the knowledge of were the girls were heading may have slipped out from the family inner circle.