I have long believed this to be a demented pack of miscreants fueled by drugs, metal music, and underworld worship. I 100% believe he/they set out to find a victim that night, and did. Whether they planned it to be Chelsea, I don't know. I think my hypothesis can be discarded or validated based on what was found in that backpack, and what else they find upon further exploration. This is a sick and evil pack of miscreants.
I always thought it was a sexual encounter gone wrong, I don't think she left with Clay unwillingly but I don't buy his story of rough sex gone wrong. I think it was more along the lines of him going too far and assaulting her and then being afraid she'd talk and he'd go to prison for rape. I highly doubt it was planned or anyone else was involved because at this stage of the game he has nothing to lose and everything to gain from throwing someone else under the bus if another person were involved and I'm sure he would in a heartbeat