In the 1970's, the darker side of the filmmakers mindset sold some tickets. There was a slew of movies where a crime committed saw reprisal, and the films did well, either in the mainstream, or on the grindhouse and drive-in circuits. A lot of those flicks have already been, or are in the process of being remade, for better or worse. Obviously they struck some kind of chord with the viewing public.
These kinds of films, "Last House on the Left", "I Spit on your Grave", "Straw Dogs", and "The Hills Have Eyes", to name but a few were successful at the outset, and saw second lives on VHS and eventually DVD. Deeply rooted in Ingmar Bergmann's "The Virgin Spring" mentality, they were classic revenge flicks without the "revenge fantasy" element. Too dark for all that. They also were difficult to watch at best, and each trumped the other in the controversy department.
Still, critics and sometimes the filmmakers themselves labelled the movies as an "examination of violence" and claim the statement being made on celluloid here is that the gap between the barbarians and the "normals" and what they are both capable of doing is not all that wide.
There may be some truth to that, but to compare the awful to the innocent is a mistake and somewhat foolish.
It's all about intent.
After that, all bets are off.
What these so-called experts don't see is the behavior of the protagonists in these movies is merely survival or retribution. These people who have been so grievously wronged act in one of two ways:
1. Self Defense
2. Levelling the playing field.
These two things are a far cry from the depraved homocidal or psychosexual acts of the antagonists. You see, by virtue of their existence, they are not the instigators. Evil is as evil does, it's not a reaction.
To group murderously evil people in with folks on vacation (The Hills have Eyes), or preparing for a daughter's birthday party (Last House on the Left), is not only off base, it's overly cynical.
Because after the score has been evened and the dust has finally settled, The victims will attempt to put back together the pieces of their broken life, not look for their next victim.
Therein lies the difference.
Good and Evil/Black and White.