Spoilers:
To me, I thought it was a really dark look at human's, at times, delusional, and savage desire to survive. I loved the direction, the score, the acting is phenomenal, the set design, those images of a young boy, a man, wearing gas masks and carrying a gas can, are powerfully unsettling, tense, and eerie. The whole movie is really an update, kind of homage, to the kinds of themes Romero brought in "Night Of The Living Dead". What would it be like when the zombies came? When the disease came? It's imperfect, but a dark, ugly movie, that traffics in the worst of human impulses and evil behind our shared values.
The movie is uncomfortable throughout, as seen through the eyes of the adolescent, and his childlike, tragic, spying, his emerging sexuality, and partial hero worship and envy of Will. Edgerton's character is wound so tightly, ever scene seeps with tension amidst the knowledge that this act of stability can't, and won't last long. The visual and direction techniques to highlight this were amazing, from the rooms lit only by lights at the end of rifles, to the sinister look of the plastic covered walls, like a Juarez kill house, families huddled against them, awaiting their death. The tension simmers, the sexuality threatens to unravel this thinly disguised nightmare, with routines like family meals, and wood chopping. It's a bleak movie, and the film may have needed some more scenes of intimacy and tightening the noose, it all seems to go awry rather quickly, which may be the point.
You are left with a couple in bloody clothes, atrocities in their yard, a dead son, a dead life, are they really alive? Ultimately the questions is, are these people, are WE people we want to survive "the sickness", "the plague with"?
for it. 
