Some early thoughts -
1. The Attacker 100 pc came ready to commit a crime , given the choice of attire, attitude presented (strange behavior noted by other walkers on the trail) , gloves on a hot day (when walking a trail, not climbing etc.) , and most importantly the taped over license plate. Ruling out spur of the moment rage , or random confrontation leading to rage.
2. Drug pick up /drop would be closer to dusk not middle of day ??, not so close to visitor center, also if the man was there to pick up or drop off something why would he be seen frequenting that trail for a whole hour or more , as seen by witnesses as early as 1.30 pm . He was walking around and up and down, scouting for a victim
3. I believe he was there to take a child, a national park makes sense from that perspective - people these days are getting far more cautious re letting kids play outside on the street, local parks without any parental supervision , ditto walking back/forth school. He gambled on the fact that families with multiple kids, let them run ahead (try and stop a kid from doing that

, I spent the first ten years of my kids life trying to run after him, till he got too fast and I too slow).
4. So lets say he got there on the gamble that he could take a kid, he would not want to injure or kill the kid so maybe he planned on gagging the kid , or sedating it (I don't know is that possible only in movies the whole chloroform over the mouth thing we see on screen) and putting the child in the duffel bag, not too far from his car for a quick getaway in the few minutes a child might be missed.
5. Cudnt find a younger child so went for the 7 year old. It went wrong and I do believe the parents took fatal blows to give the kids enough time to get to the parking lot and beyond, maybe trying to hold down the perp or something forcing his hand on them to the finish, I truly hope he was injured.
If this was not the random scenario above, and someone they knew / a personal killing than I have a feeling (as theorised here already) that it might be someone in the oldest child's sphere as she was the one not there that day. But again they would then pick a day where none of the kids were there ?