The timing of all of this is just too precise for RA to have gone out at random and killed at random, IMO. RA's vehicle was likely seen at 1:27, which had to be within what...10 minutes of the girls leaving L's house for the trail? He gets there, parks, and walks directly to the bridge, again, within minutes before the girls get there? They are half-way across when they post a couple photos around 2:07, with RA nowhere in sight. Six minutes later he's behind A on the south end of the bridge, almost to them.
It's been said that a third friend, and maybe KG, were invited to go along to the bridge that day, but neither did. If this is true, I don't think a meetup with a boy was necessarily planned, and the trip to the bridge very likely could have been last-minute, as the family suggests. However, I'm very curious if L posted anything on SM that morning, shortly before they left.