What do I say... what an unspeakable tragedy this is!
I wonder what made Libby take the video/audio?
If they felt the danger from the distance, could they run away, hide? Did they make any attempts to call 911 or family?
That “go down the hill” sounds so controlled and deliberate. I wonder if this evil monster had a gun to control the girls...
Do you think this was a planned attack or wrong-time wrong-place situation?
Apparently they sensed some danger at the time (family members who have heard more of the unreleased audio have said in interviews that the girls mentioned the man) but perhaps it didn't rise to the level where they thought their lives were in danger. Personally I believe that their gut told them something was off about the guy but they had no idea what was truly about to happen.
We know from the sheriff's recent Q&A in The Carroll County Comet two things that pertain to your questions. First, the sheriff says that the killer was able to gain control over them on the south end of the high bridge by a combination of manipulation and intimidation.
Secondly, the sheriff described the girls as "victims of circumstance or opportunity" in that same Q&A. So that would seem to indicate that the fact that they were killed instead of anyone else at the trails that day was a wrong time/wrong place situation.
I would say, however, to your specific question, that "planned attack" and "wrong time, wrong place" can coexist in the same crime. An offender can plan his crime in great detail but not have a specific victim in mind, rather a specific day and place where he feels it will be easy to find the type of victim he wants. He can plan the types of things he will do to a victim in great detail but not be too picky about knowing who the victim is in advance.