Motivations (SPOILERS)
It's interesting to me that, at the end of the film, they make it clear that they brought sacrifices from outside the village (well, not that this wasn't pretty clear from the outset), and that they needed nine sacrifices from in and out of the community.
But, I'm thinking back on the film and several of the "sacrifices" seem to be motivated by other things. Connie and her fiancee seem to be killed because they're freaking out. Mark pees on that tree, and Josh is taking pictures of the sacred book.
Would they have died anyway? I assume the answer is, "Yes", but then why bother killing - for instance - Josh at that specific time?