ETA: We’ve talked a lot about profiling aspects, etc. I think I’m finally ready to take a poooosssibbble leap to say that maaaybe one of his chosen and efficient MOs is to kill on-site. This doesn’t necessarily mean he is not one to abduct from a scene, but perhaps this a good and comfortable strategy that works for him. Moo.
SBM.
Would you think I am totally off if I post another idea that has not been discussed here?
The question is more to those who might know something about the religious aspect of this case. I am torn about the religiosity of the BG, I feel he is too emotionally shallow to be even that. However, he must present as a religious person, otherwise I can not explain Carter's hints, the shack, and mostly, that the girls are not in the way BG had left them. And this brought up a question of how they were left, and that, in turn, brought up another idea. Now, I don't know the exact circumstances the girls were found, I can only guess what has been already discussed here and probably is OK to mention as it is BG's MO - some form of posing. Done by a delusional man who is unafraid to end in hell for killing two teenagers, btw. Religious, afraid to end in jail, but unafraid of hell. This is something...
So I thought... can it be so that BG is trying to recreate some religious scene? After he killed them?
That could explain why the clearing (how these things are often painted in medieval paintings). In fact, he needs only one person for the scene. And I would surmise, the one that has long hair and looks more medieval-like. He does not need teenagers as such, but someone who'd resemble a very young lady.
I think he knew the girls, and they might have seen him. But still think that in a way, they were a random choice. I honestly don't believe he is known enough in Delphi for the girls to have his darkest secrets. At best, he is unnoticed. I think he probably was not scouting for them personally, but for someone, one person, who would do for his plan, looks-wise. (And that the second person was the hazard of his plan, sorry to say).
Did the photo play a certain role there? Maybe, or maybe not, it could have shifted his choice.
The fact that he was lurking there for several days, signifies, I think, that he was waiting for someone who would look like she'd fit his purpose.
Delphi and its environs are well-known. Hence he ended up here. Next time he'll go somewhere else that is, as
@margarita25 has said, private enough, but naturesque and flat.
I don't know the details of the murder, but I wonder if Catholic medieval paintings have elements of what he has done.