ok here's a weird what if for Shannan...
could she have ripped off JB for something he didn't want mentioned to police?
could she have hidden the drugs and called the cops to get her out of there?
it could explain her over the top fear... She was at JB's for quite some time...he says they didn't have sex..so what was going on? why did she think they were going to kill her?
could she have been trying to pull a fast one?
Its possible. It could be that she wanted paid and figured JB would panic and fork over the money to get her to leave if she called the police. Saying someone is trying to kill you would definitely up the priority of the police response. I also wonder if there was some bad drug interactions with medication or maybe the batch of illegal drugs was bad or didn't sit right with her. Even some really potent weed could make someone that smokes regularly wig out if the setting is wrong.
Two strangers, a creepy run down house, and a community on a remote barrier island could all add to the negative setting. Add to that a stranger that is getting pissed that you won't leave because you want paid for something you didn't do...