Blue Ruin 
Perhaps not a "horror" in its purest summation, Blue Ruin is as terrifying and hands over your face scary as any recent jump-scare ghost flick, The Conjuring and We Are Still Here included. Add in some bursting-flesh bullet kills and foot on the pedal pacing, it could rival any of the recent ultra-violent crime thrillers as well.
The homeless and mentally scarred son learns the killer of his parents is being let out of prison via plea deal, and well holy shit he just won't stand for that. And that's literally all I'm going to tell you about the plot. I'd even advise you to skip the trailer...the least you know going in, the better.