It ain't horror, but it has a body count, and that's good enough for me.
That, and I absolutely LOVED this movie. Falling Down is one of my top fives of all time, and the plot is VERY similar, although this is more dark humour than intense drama with a bit of action. You get more into the gunman's head in this movie, as he is less of a cipher, and the world is so much worse now, arguably, that it gives them a ton of social commentary to make.
Highly recommended to any fans of black comedies, and Falling Down.