I think he was cold and calculating. I think his body language was calculated, he intentionally tried to look like a non-threatening guy who was just walking. Nobody for the girls to pay any attention to. He probably just walked behind them for a while and seemed to not care that they were there. He would walk near them for a while to make sure they were all actually alone and probably to wait until they were near the end of the bridge so they were easier to control and, again, he could be sure they were alone out there.
His voice was cold, calculating and calm. He obviously has good self control. He didn't ambush the girls. He seemed to assess the situation and spent time observing them before making his move. When he made his move, he didn't lose his head. He controlled the situation, he didn't yell, which would have alerted anyone in the area. He calmly but firmly ordered them down the hill. He waited until he was in a secure location, which he had probably scoped out, to finally do as he pleased.
I am wondering about how he got two smart and athletic girls to do as he said though. We know Liberty recorded him so I have to wonder if he harmed Abby and then used her as bait to get Liberty under his control. It's an awful thought but I don't see Liberty following his orders for any other reason.
There are residences closeby, too.
Another thing that bothers me about this case, and it fits in with what a lot of have theorized about the timeline (and it's a short one), is the perp knew they would be found in a matter of hours. They were found roughly 22 hours after the SnapChat was uploaded. Not days, weeks, months, or longer. Hours.
Something I'm wondering about the SC pic of Abby:
Like others have stated, it does look artistic, maybe posed. It's a really nice photo. MOO, they were taking their time getting across the bridge, especially that SE end, the pic is taken, and let's say Libby does a quick effect or edit to it, before uploading it. Or maybe there were multiple pics, and she scrolled through them, found that one, touched it up a tad, then uploaded it.
They could have been off the bridge, already, when the pic was uploaded, in fact the perp may have been making his way across at 2:07 PM. Libby is already in her phone, anyway, scrolling through pics, uploading one or two, when the perp gets on the bridge.
They were dropped off around 1PM. SC pic is uploaded at 2:07. Judging by the distances involved, from the end of the trail to the SE end of the bridge, the girls could have made that distance, even walking slowly, in well under a half hour. So either the pic was uploaded instantly (unlikely), or Libby uploaded it at 2:07 PM.
MOO is the guy was on or close to the other end of the bridge at around the same time as the pic upload to SC.
Edit:
Judging by his steps in the images, it probably took the guy under 4 minutes to cross the bridge.