So true--the murderer(s) might be criminal masterminds, or might be lucky morons--I'd guess it's some combination of forethought and luck, but you're right, we have no way of knowing that right now.
I think you're right about the scouting trip. Remember DWW (dog-walking woman), who saw a mysterious person at the far end of the bridge around mid-day? My own guess is that BG scouted from the trailhead to the far end of the bridge. DWW makes me think it was at least a couple of hours before the murders.
In my guess, BG watches DWW leave; then he strolls from the south end to the trailhead (remember the theory that the girls saw him near the trailhead and spoke?) Now he knows there is nobody behind him, on the bridge and to the south of it. He then waits/lurks somewhere on the trail. Now the girls appear on the trail and cross the bridge. He gives them a few minutes, then follows them across the bridge. Because he has been scouting and monitoring, he knows there is nobody anywhere in the area, and there is no easy escape from that end, either. The girls are alone and vulnerable. Tragedy ensues.
This sounds likely to me (MOO). Whether the girls (or just libby) were previously targeted, or if they were random targets of opportunity, isn't clear but it could be either, in this scenario. Planning plus luck? As the man said, "One day, we'll know."
