This is a random post, but I literally cannot stop combing over every possibility of BG in my head. From SK to organized crime, I feel like I've tried to hash out every scenario I can imagine. Sorry that this is mostly just thinking "aloud," but I feel most strongly about this personal theory:
I think LE has a good amount of information/DNA/etc. to definitively link a suspect to the crime, but has no idea where to even look for said suspect. I feel that he is local, that it was a crime of opportunity and he got extremely "lucky." I think BG is a quiet loner in or near Delphi. I suspect that the crime itself is captured audibly on cell phone and that is what has shaken LE so deeply. I also feel a "kill" was not the intent, but the unfortunate end result of a somewhat impulsive attack on two girls. I think it was a quick crime with a quick COD (gun with silencer, perhaps).
Although unseasonable warm that day, as described by residents, it wouldn't be odd to cover up in the way BG is, in my opinion. I think the fact that he had so much clothing on gave him an opportunity to conceal a weapon should he be able to use it on the trail that day. I figure he probably often took a weapon out to the trails in case an opportunity arose.
In summary, I feel BG was ready to do this at some point and February 13th happened to be the day that the opportunity presented itself. I don't think that even he could have predicted how easily he'd get away with this thus far.
All my opinion only.
I really would like to hear counter arguments with proven discrepancies so that I can discount this theory (and work on others, no doubt).