I tried to watch the original "Last House on the Left" when I was much younger, and I wasn't able to finish the movie. Recently I watched the unrated remake, and I regret it. Some scenes in the movie were far too realistic and sadistic.
I am a horror movie fan, but I am not someone who enjoys watching humans behaving sadistically towards other humans. To me these are just torture movies. I enjoy horror movies because I know that they are not reality. What scares me is the unknown, ghosts, monsters, zombies, things that are of a religious nature such as in the Omen movies, viruses like in Quarantine, aliens...things that are mostly based in fantasy.
I already know that the human monster is real. Movies like The Last House on the Left, Saw, The Texas Chainsaw Massacre; The Beginning (which was the most sadistic, violent movie that I have ever seen) are not what I enjoy. They don't scare me so much as they make me sick to my stomach.