"I don't know anything about anthropology" seems like an appropriate response to witnessing murder. How many red flags do you need god dammit.
Anyway great directing, and stellar acting from Florence Pugh.
<spoiler>The movie burns the doubt card too fast to hold any tension, which was kind of a disappointment, then decides that it wouldn't be a survival but a slasher, which was another disappointment, and ultimately chose to be cynical, which was a final disappointment (I'm sorry but building such an empathy-inducing character so that she can become the queen of psycho-swedish barbarian dickheads is beyond my threshold of tolerability). It still has value in its study of the mechanics of a cult, all wrapped up in a bizarre but captivating atmosphere. It is, in fact, this atmosphere that makes me like the movie in hindsight. Rarely a movie has left such a striking aftertaste in my mind.</spoiler>