Okay, to murder someone is never logic to me, but I really can´t understand this: If the suspect was out there to kill (and maybe rape, which we do not know), why would he decide to murder two girls? It seems to me this was very risky for him, because even when they were not as strong as him and he had a weapon, there was a possibility at least one of them could run away and call for help. So, if we try to think like someone who is out there to murder: Why did the suspect not attack a woman who was out there alone? Or alone at another place?
I really do think he knew them, not neccesarily from beeing friends with them or having to do with them on a regular basis, but at least from distance.
Because a total stranger would not see two girls and think "Cool, I am going to kill them" - then if he would, he would have killed lots of times before, wouldn´t he?