Demons vs. sinners
From a theological standpoint, I (as well as many other people who have seen this movie) struggle to make sense of what exactly constitutes a "demon" in the context of the film. We are given flashbacks that show the horrible acts that Dad/Adam's victims apparently committed— all of them were murder (as well as rape for the pedophile/child killer). This is where things get foggy for me though in terms of what the subtext here is actually implying.
I was raised Catholic, and to my understanding, demons are inhuman and entirely separate from man. They are evil and have the capacity only to do evil. What I didn't understand about this film was that the line between mortal sinner and demon is indefinitely blurred. Yes, all of the people that Dad/Adam "destroyed" had committed horrible murders, but what about that makes them "demonic"? Some have argued that the implication is that some people are simply born evil, that they're bad— they're demonic— and those are the "demons" who Dad/Adam were assigned by God to destroy. The problem I have with this is that, although these people committed awful crimes against other humans, they are in fact still human themselves; they show no signs of being quote-unquote demonic according to the terms of demonology, but rather people who committed grave sins against their fellow man.
Is the film attempting to reconfigure the term "demon"? Is it positing that actual "demons" are merely those humans who commit grave sins against others? The writer's treatment of this term is my only real gripe with this movie, as I found it on the whole well-shot and utterly captivating— I'm just very confused in regard to what it is attempting to say about the nature of demons.