Horror?
I'm not really sure that I would call this or any other movie of this type, (serial killers), horror movies. Disturbing yes, brutal yes, thought provoking yes, disgusting yes, depressing yes, etc... But not what I really consider "horror".
Sure, we can all be horrified by the possibility that someone like this lives next door to us or down the road from us, or even worse, in our own families. But, when I think "horror movie" I don't consider movies like Pulp Fiction to be films in the horror genre. Which this movie is similar to.
To me, horror movies must have a touch, to a greater or lesser degree, of the supernatural in them. I'm not saying that all filmmakers should just stick to remakes of Frankenstein, etc.
You can hear/read about individuals like this in newspapaers everyday.
You know, prescription pill addicted zombies that slaughter office buildings and schools full of innocent victims. Too many people are just one more Ambien or Xanax away from sleepwalking through a series of voluntary or involuntary murders! But, that is for the courts to sort out.
Just gimme a Romero zombie flick anyday over these ripped from the headlines serial killer movies.