it's everyone that becomes their own worst enemies when they could've helped one another instead, but the lack of ressources and the climate of uncertainty and terror makes it so that people hurt one another instead, and it produces collateral damages (the neglect of the two children)
the children hurt themselves pretty badly too, since at some point the older boy gets stubborn and strays from her aunt's appartment, resulting in his sister's death
the Americans are shown as faceless justly to show another side of these conflicts, but as it stands it remains a pretty cheap way to extract sympathy from the viewers, this is a movie with a powerful message but I didn't find it "depressing"-- however mainstream audiences that aren't used to anything else than cuddly Hollywood happy endings will be extremely saddened by its tragedy