Some of my thoughts. I think BG came to the trails that day prepared to kill if the opportunity arose, a predatory lurker. I would say familiarity with the trails is a given. I do not think a possible earlier encounter that did not end well in no way should be interpreted as the girls contributing in any way to their deaths. I have myself been in the presence of people whose behavior is so far out of the normal range that it brought a gut reaction to me. Some people just emit bad vibes. This could have happened thus Libby videoing him. I think he had them targeted by the time they reached the bridge. Whether he had targeted them before the impromptu bridge trip I am beginning to change my thoughts. I now think there is a possible connection between the phone reset and the murder. I just think he had to be physically near them when Libby took Abby's picture on the bridge. I've even wondered if he could have somehow been on the bridge's understructure. It just seems too short a timespan for Libby's picture which does not show him and then his approach of the girls at the end of the bridge.
I think he was in the woods at the far end of the bridge and walked past them enough to check that nobody was coming the other way or observing from the trail and then he turned around and trapped them.