This is all speculation, and my own thoughts. Abby and Libby are victims of a barbaric crime. If Abby had arranged to meet someone she had been in contact with, perhaps over a period of time, how would they contact each other? Maybe there is evidence the police have and it's part of the reasoning of the police that it's someone local. Someone was messaging her directly. It could have been a burner phone, that was used very carefully and exclusively in the Delphi area, or notes could have been left somewhere for example. That could be seen as 'really romantic' and exciting by young girls.
I think Abby lived quite near the bridge. She may have been told by this 'boy' that he really wanted to see her, it was Valentines Day etc. Would she (Abby) meet him at the bridge? Alone.
Abby told Libby and they agreed, Abby was not to go alone. She'd go too. That's what girls do. We go to the loo together, we meet boys for the first time together. The imagined scenario gone bad needn't have been something as terrible as what did happen, just that the boy might be ugly, it might be a joke so Libby went for as much as anything, moral support.
What we know of the scene where the bodies were left, is that it was horrible. Lots of physical evidence, not the sort of thing you'd expect and Carter said that 'the girls are not in the same way you left them'. Does the horrible scene relate to the way the bodies were staged, or is it the entire scene?
If the bodies were the horrible bit, was the scene also intended to be a weird 'wooing' place? I'm thinking flowers, a pile of clean leaves to sit/lie on, maybe a teddy bear, pretty stones, feathers, a tidy and private area. A mix of items to reflect what a boy/man thinks a girl likes, whilst also reflecting the boy/man's view of what he thinks men do to romance women. Wine, chocolates? A red rose? Who knows?
Was the killer the person in contact with Abby, if so, what went wrong? Had Abby assured him she'd come alone? I've heard the girls may have seen him earlier, did they look at him in a way that indicated fear or giggle together, or share a pulled face? They didn't know he was whom Abby was meeting, but he knew at that point that his meticulously planned experience had gone. Now he was rejected (probably a familiar feeling to him) and he was angry. Or she stood on the bridge as arranged, but he'd already seen Libby was there too. She'd lied to him.
*Or, do we go with the two people scenario? The man and his similar looking son? If it's two people, it doesn't mean that both intended and/or were involved in the murders.
Could BG have ordered his son to leave and if the witnesses are accurate, wait for him at the abandoned building? He tells the son that he knows enough to get the death penalty. Maybe his mum isn't very well and if they both go to prison it will destroy her. If he tells, he's essentially writing his mum's death warrant. Or that if he tells, he will kill him, his mum, siblings etc. His son leaves, upset and angry, but silent.*
BG practically runs across the bridge, knuckles clenched in his pockets. It's very much a "Get (the f?) Down the Hill". YOU DO WHAT I TELL YOU.
The killer uses the prepared scene and adds some of his own extra touches reflecting his rage. More violent and intimidating touches, possibly part of a rape/abduction kit he carries around with him. Does it become chaotic in his rage? The scene becomes very confusing.
The killing may be the thrill for him, (or, if there were two people involved, the sexual intention was ruined by arguing with his son). His method of killing may be time consuming. He has to choose between a sexual element and living out his fantasies. He can't be too long, people have seen him, the local residents might see him, or perhaps he heard someone's voice and it unnerved him.
But once he's killed, he's calmer. He stages the girls, perhaps reflecting his rage with one of them, gentler (and I use the word with the knowledge that that sounds oxymoronic) with the one he had such plans with.
