I am in the random-yet-not camp. I think the perp has thought about such crimes, probably having attacked in the past (not so sure if he's murdered before or not, but it's a possibility). I think he fantasies and has scenarios in his head that he can carry out when the opportunity presents itself. And, I think he places himself where those fantasy opportunities are more likely to happen, like on a trail on a school holiday. If the opportunity doesn't happen, he just goes on with his life. If it does happen, he kicks into action acting out his fantasy.
After that I think he swings back and forth from elated to exhausted for awhile, then settles into a calm period....which then builds to another period of being ready for opportunities.
Yep, armchair analysis, worth what you paid for it.
jmopinion at the moment, subject to change as we learn more.