Gender + ethnicity in Nightcrawler
The director/writer considered race and gender issues in the US when writing: the news was always after the contrast, e.g., white females who were burglarized by blacks from outside the suburbs.
Thus his choice to cast a black female as the only character that is truly on to the scope of Lou's ("white male") moral perversity/criminality strikes me as significant, especially given her impotence to prove his crimes. Or that Lou's exploited assistant, Rick, is latino-looking, in a state that exploits hispanics to the end in agriculture. It's almost over the top in its metaphorical meaning.
Add to that the subtext of Lou being sexually coercive/rapey/open to domestic violence, and he's an amalgam of all the abuses of power that a person can legally get away with in modern society.
It's pretty clear that the movie is a (phenomenal) allegory for unconstrained capitalism. Fantastic satire and critique of society.