Disclaimer - All of my following comments are my opinion only and should not, in any way be confused with being factual.
I have theory of a possible scenario. Please share your thoughts, alternative theories, poke holes in mine, etc...
Here goes... So the girls are out hiking and taking pictures and video. From a distance, the unsub notices them and is not happy about where they are. Maybe he has noticed them, or others, in the past and just does'nt think they should be where they are. Or maybe they are doing something that the unsub is not happy with. (Imagine, if you will, of the crotchety old man who yells at the kids to "stay off my lawn".) The unsub is "fed up" with seeing them or others where he believes they should not be or doing what he believes they should not be doing so he decides he needs to confront them and eventually catches up with them at the Monon bridge where he is captured (in the background) on video from the girls' cell phone. Once the unsub catches up with them and confronts them. He may have forced them back to where he thought they shouldn't have been or what they shouldn't have done to "make his point". Sometime during this encounter, one or both of the girls respond in a way that causes the unsub's anger to escalate. Eventually, he loses control and attacks and kills one of them. Then, he realizes what he has done and decides the only way to avoid being identified is to kill the other girl too.