Before the image of BG was released, before the audio portion of the tape came out, my gut instinct was that it was a peer-related death. Some kind of hazing or bullying incident. Due to the fact that the girls were alone, it was a day off from school, a place where other kids their age sometimes hung out, etc. I thought they might have met up with people there, or simply ran into people there, and something REALLY bad happened. I thought one of the girls might have even been killed by accident and they panicked and killed the other so as not to leave a witness. That was my first instinct. Bullying is so bad these days and cyberbullying is even worse.
Then the audio/image were released and BG was presented. That scrapped my whole original idea.
Going back to the beginning and starting over, I DO wonder if this crime is a lot simpler than we think. When an arrest is made, it won't surprise me at all if we sit back and go, "Oh yeah, wow! Really should have seen that coming..."
With that being said, I don't have a specific person in mind at all.