I'm occasionally asked what is the most disturbing film I've ever seen. This one is easy because there is absolutely no doubt: "Salò" (aka "Salò, or the 120 Days of Sodom"). Directed and co-written by Pier Paolo Pasolini, it's focused on portraying graphic atrocities in Italy during WWII. It is widely considered to be the most notorious professionally produced film ever made, and has been frequently banned (in some cases to this day) in various countries. I've never walked out on a film as an adult, but it is probably the only film that I almost walked out on during a theatrical screening.
Sidenote: Pasolini himself was murdered three weeks before the film's release.
