My first theory was that this was an attemp to abduct the girls (or one of them) that went wrong. To me, this seemed like a perfect spot to do this. A road under the end of the bridge and all he had to do was get the girls Down to the private road and into his car that would be parked somewhere there and then he could be out of the area pretty fast, as the private road goes straight to the bigger roads in the area. I did a bit of reasearch as to whether this would even be possible, if there was a locked gate where the private road start, Security cameras etc. However, when the girls understood his intentions they tried to escape and he chased them to the other side of the creek, where he killed them.
The more I have looked into and learned about the case, I think this could also be a very planned crime, where he intentionally led them across the creek with the purpose of committing the murder at that exact spot where they were found. I don’t think the girls were targeted, though, but random victims that just happened to be there and he saw an opportunity.