Here are my two takes on Erin's murder. They each involve both Ross boys:
(1) Billy is the father of Erin's baby and John knows this. Billy and Erin were meeting up that night to hang out, drink, have sex. Erin somehow tries to get money out of Billy ("I'll tell" threats, she charges for it now, etc.) for DJ's ear surgery. They are drunk and argue and the argument escalates to physical -- maybe she runs away and he tries to shoot at her but is too drunk and that's why her finger is shot off and the bullet in the tree. She falls and hurts herself badly. He grabs her and is holding her trying to shake her awake or something, which is how he gets covered in blood. Drunk, not thinking clearly, he calls John for help. John says he'll get rid of her body but she wakes up or stirs in the truck, and John kills her and dumps her in the creek but lets Billy think she was dead and Billy is the murderer.
(2) Similar approach but with John as the father. Erin wants to meet with John but he refuses and sends Billy in his place, with a "take my gun, just in case." Argument ensues, she runs, he shoots at her and misses, and calls John. John tracks Erin down, kills her, returns to a drunk Billy and convinces him he shot her when she was running. In the days following, John continues to psychologically manipulate Billy to solidify the story in Billy's mind.
RE: Dylan -- I think its more likely he's a drug dealer than a pimp -- remember the girls were putting themselves on a website, which enables them to control who they choose to sell their bodies too -- it eliminates the need for a pimp. Unless the third friend has the skill to run a website like that, Dylan isn't smart enough to be behind something like that. He's absolutely a sociopath but I don't think he's a pimp.
S01.E06: Sore Must Be the Storm
in Mare Of Easttown