Anybody else ever consider that "guys" is not used in the same sentence as the words "down the hill" in the recording, or that "down the hill" was NOT a directive?
Sorry, a long post. I'm not very good at being succinct!
I think it's separate. I think it could something along the lines of "Guys, do your parents know you're on the bridge", or "Guys, you shouldn't be on the bridge, it's dangerous." At which point Abby and Libby look around and see how steep it is and/or have always gone back across the bridge and say "How do we get back from here?" His reply is "Down the hill".
If they did recognise him, or just followed his instructions because he's older than them and is a sort of authority figure, they could have reluctantly, but willingly gone down the hill. It may be that he didn't start the attack until they were at the bottom of the hill. At that point it was much easier to control the situation.
It's occurred to me that, despite filming BG, the girls could have felt more comfortable at this point. This would work particularly well if they did know him, or if he did look like the sketch of the younger man. If he did, then he's relatively attractive and it wouldn't be that surprising if the girls consequently dropped their guard.
Could he have pretended to hurt himself going down the slope? The girls could have felt compelled to help him. He could then could have grabbed Abby and put a knife or gun at, for example Abby's throat, or even knocked her out. If this happened, the shock would possibly silence Libby, long enough for him to tell her if she screams he will kill Abby.
Some of you may think this sounds unlikely. But I am 40 and if I was in the scenario I have described, I would be very likely to do just what I have suggested Libby and Abby did. Not because I trust everyone, but out of politeness. That sounds ridiculous and maybe it's because I'm English and we're renowned for our manners, but I think it's possible that this happened.
One final thing to mention. If I have understood things accurately, everyone else who has admitted to be at the trails that day was local. Maybe Abby and Libby had never seen anyone there, on any of their visits, who wasn't local. I live in a reasonably small place (though larger than Delphi) and I have a sort of assumption of trust regarding people round here.
If I was 20 miles from home I would be more on my guard, but at nearby beauty spots, I would feel safer. It's a combination of a familiar place, years of visits, always seeing locals and the fact that nothing bad has ever happened there. I'm really not as naive as I sound, but round here strangers say hello and exchange a few words. Remember Cheyanne said hello to BG. My final thought is that did BG look at her in a scary way because he didn't want to be seen, or is he not used to complete strangers saying hello? If it's the latter, I'd guess he doesn't live locally. It's simplistic, but sometimes things aren't as complicated as we try to make them.
JMO MOO