I snipped the post and bolded some of your comments because it made me think of something. Maybe off in left field. It goes back to Supt Carter asking in an early PC "Why Libby, why Abby, why Delphi?". I want to preface this by saying I do NOT believe Delphi is a bad place, rather I am speculating that is how BG perceives it.
So, what if a vulnerable human was not his only target, what if Delphi was too? What if BG did live or work or go to school or have some connection there. Maybe his childhood there was bad. Or he landed there but it wasn't his choosing (a move, a school or job transfer) or it was his choice but ultimately it didn't meet his expectations. Maybe he wasn't esteemed as he thought he should be, or maybe he was trying for a fresh start but his problems followed him. Something has not been right with this person for a very long time. What I'm suggesting is event(s) occurred in Delphi that left him feeling even more powerless, angry and resentful. And through association he blamed Delphi for his escalating misery and downward spiral when in reality it's something inside him. Maybe he wanted or felt he deserved recognition, he wanted to be remembered, he wanted notoriety. But he wasn't getting it through normal channels. And that enraged him.
My point: did these murders serve a dual purpose for him? Were there two targets: human and an entire town? Was he satisfying his urge to kill and getting revenge against Delphi at the same time? Does that answer the 'why Delphi' part? He's not a 'nobody' anymore and he made sure Delphi (his other target) will never forget him. LE says it's about control. He controlled two young girls and took their lives, but he's also manipulated and controlled the emotions and behaviors of an entire town through terror. Just a thought and MOO.