The innate inhumanity in humanity (spoilers)
This film was disturbing, upsetting, and enraging, but I did like that it touched on the innate evil in us all. Of course our thresholds and limits vary, and some cross them easier and more willingly than others (like Sandra, who always wanted to please authority, and undoubtedly would have been a Nazi).
I thought the ending where Sandra insists that she was a victim was a key. After being negated, she then dissacociates/changes the subject when confronted with her true nature on tape. She seems like a sweet, matronly manager...but there is something inside of her, something that goes farther than nativity and ignorance, in my opinion, that this man is able to pick out from her and set into motion.
It was strongly hinted at that Sandra was jealous of Becky, and it was quite clear she enjoyed being praised and having her ego stroked while a young, pretty girl she ended up willingly convincing herself had stolen something (despite ALL of the evidence to the contrary)
It no coincidence that Becky was the one who...*SPOILER*....was raped by her fiance.
To me, this "Officer Daniels" orchestrated a community rape. That's what made the film so deeply unsettling. It wasn't just that Becky was the victim of one person's ignorance, or a "prank call," but that everyone in some way contributed to her torture and assault.