Having to murder people over and over again to save your naive and childlike brother's life is truly horrifying, so yes it's a horror film. It's a horror film that goes beyond to the blood and does not glamorize the violence. That makes it more horrifying.
I would agree it's also a drama showcasing the tension between the brother who must "provide" for his family and his sister who calls the shots, and believes killing inferior people to keep her younger brother alive is justified.
It's a heart-breaking story, and you find yourself rooting for Dwight, the older brother, to escape this never-ending hell of a life situation.
Like so many good horror films, grief is the root of the horror.
A horror movie with HEART. It was harder to watch than a typical horror movie because of this, but it is a quality film.