A good horror film can genuinely scare the viewer whether with the narrative, the suspense, the monster, or the gore. More often than not, scary movies miss the mark, and any horror buff could tell you there are plenty of titles that just don't qualify for the genre. The most disturbing horror movies are the ones you have to watch from behind a pillow or through your fingers, these are the films worth turning on at midnight with the light off, the ones that would make even the most experienced fear seeker cringe a little.
Whether you draw the line at cannibalism, body modification, creepy kids, or the devil himself the horror movies you can't finish likely highlight one or more of these common aversions. Disturbing horror movies push the limit on what's actually scary. For Raw this means relating to and sympathizing with the cannibalistic villains. For The Human Centipede II this means watching people live through unimaginable suffering for an extended amount of time with little to no hope of recovery. Whatever the premise, some films are too horrifying to watch all the way through, so much so that perhaps only a true horror fan could actually finish them.