I get what the movie was going for - depict how far someone will go to protect their own kin - but I was left a bit disappointed. I might sound like a monster for saying this, but I did not feel (like the film wanted me to) the tragedy of the child being killed towards the end because it felt like a logical extension of the methodical disposing of the grandfather at the beginning of the film (under the assumption that the Joel Edgerton's character believed the child to be affected by the disease).
Something that comes to mind is the first part of Dekalog (Dekalog spoilers ahead) where a child dies because of the father's reliance on a computer program to tell him if a frozen lake was strong enough to hold the child. In this case, the tragedy was completely illogical, unanticipated, and completed unprecedented within the universe of the film. I think that is the right way to go about it.
Of course, in the real world, the death of a child is always tragic, and it is almost always represented in cinema as such, but for the purposes of It Comes at Night, the moment was not affecting enough to carry the weight of the "family unraveling" theme. At least in my opinion.
I did like the idea that the teen boy seemed to be affected throughout the film and his parents didn't take notice while when there is the possibility of the other family's child being sick, the main couple freak out. That aspect of the "family first" dynamic hit home even though I think the climax discussed above did not.
Cinematography was just as spectacular as nearly every other A24 film I have seen, and acting was far above par. Writing, on the other hand, was somewhat heavy handed; for example, when Will accidently calls his brother-in-law a brother, implying he might be lying about his past, I immediately identified it as a red herring because I had already recognized the film as one of those films that is intentionally unsatisfying such as No Country for Old Men (resulting in no resolution given to this flub in Will's dialogue). That last sentence might have not made any sense, but I'm leaving it because it took me too long to type.
Also, it reminds me too much of 10 Cloverfield Lane for comfort.