It is so scary and chilling to think he just crossed the bridge with such confidence and deliberation. And at that point he knew exactly what was going to happen while these young girls stood there in fear and uncertainty.
Lots of assumptions there. I tend the other way. He had no idea what was going to happen. His mind was a mess with a thousand things going on. These guys don't kill every day. He may never have killed previously. If not, then all the years of contemplation and build up are finally at hand. That's why he's kind of in a head down daze while approaching the girls. I doubt he fully expected it to happen until just about that point. Prior to that he's fully prepared for an interruption, like somebody showing up on the gravel access road below, or the girls dashing away. He may have had interruptions on prior attempts elsewhere. Once he realizes all systems are go then he's figuring out what to say and when to pull the weapon.
All of that combines to not being fully aware of Libby filming the video. I have always been shocked that the situational reality Bridge Guy is facing is so easily set aside, in deference to assumption of a calculating killer who envisioned every stage of that day exactly as it played out.
Otherwise, the odd aspect for Abby and Libby would have been someone else on the bridge at the same time period. That's going to dominate their mindset, far beyond fear. Libby in particular would have grasped how rare and ridiculous it was. But since she didn't recognize him she mostly flicked it aside as a weird tourist who didn't know any better.
I sense that Abby became somewhat concerned while Libby was rightfully anticipating nothing more than a brief awkward encounter. Abby apparently asked about options. If Abby had insisted on running away I think Libby would have done so, despite the trespassing warning. But keep in mind the extremely tight time frame. They maybe had 15 seconds after Abby stepped off the bridge before Bridge Guy reached them. Libby's already turned around and facing Bridge Guy while standing just beyond the bridge. It's a strange situation but not one to automatically promote fear. I know darn well I would not have run away. I spent 24 years in Las Vegas and the median time I returned home was probably 2:30 AM. That's countless encounters with crackpots of every description on the Strip.
I can think of one instance in my life where I scared somebody into dashing away. I was just about Libby's age, maybe 14 or 15. I took the day off from school to attend the PGA golf tournament at Doral. My parents dropped me off. They couldn't pick me up at the end of the day so I volunteered to walk the 6+ miles. No big deal. It sounded like fun and was fun. Like an idiot I wore my metal spiked golf shoes. That seemed like the cool thing to do in that era, dressing like a golfer while attending a tournament. Anyway, I was within a mile of home and it was now dark. I'm walking briskly on the sidewalk. You can imagine what that sounds like while wearing metal spikes. There was a guy in front of me, roughly my own age. I'm maybe 30 yards behind him and gaining since I'm tall with a long stride. He crossed from right side of the road to the left side. I did the same, on almost the exact path. Not because I was following him. It was pre planned. That was a logical area to cross, given the major cross street not far away. But as soon as I crossed at the same time he did, that kid took off like a scared jackrabbit. It took me a few seconds to realize what happened. Wait a minute...you're frightened of me, a kid with golf shoes? I'll never fault Abby and Libby for not dashing away, because virtually without exception that's what they would have been running away from...nothing.