Dropping my theory here before the PC tomorrow.
At this point in time I'm returning to my day one theory. Lone wolf who didn't plan anything, but saw an opportunity with the two vulnerable young girls and took it, probably targeting Libby more. I feel he made harassing remarks at the girls, and they felt unsafe, which led to Libby recording. He got furious at seeing Libby recording, forced them down the hill with a weapon (possibly gun) and used a hunting knife. This is strictly my opinion only.
While I think the killing was unplanned, I do wonder if he had seen one or both around and possibly became obsessed with them. I'm sure he knew school was out but I don't think he went to the bridge strictly to look for the girls.
At this point, until LE confirms, I don't feel that KAK is connected. We will see later and I am probably in the minority on this, but I think these girls were unfortunate enough to encounter many creeps. As someone who was once a teen girl, it makes sense.