Whatever you call The Mist and Silent Hill (the movie)...casual cruelty I find very horrifying. There's a foreign movie that I saw about a Christian missionary in either China or Japan. I think she was taken captive by a shogun or samurai...the torture did it for me.
There's two instances in fiction where I've been really scared, though.
One, in The Dark Defiles: The hero comes home, frees a bunch of hardened criminals and gives a speech that whips them into a frenzy of firing, massacring and rape...it is basically him releasing years upon years of pent up anger and abuse. Abuse and anger rooted in him having been raped as a child by fellow knights-in-training despite the cultural norm of violently executing homosexuals, and in seeing his old lovers experience public execution in the form of being impaled and left hanging in a cage for all to see, watching as children threw rocks and mud at the impaled victims...it all came bubbling up when he returned home.
Another, in The Cold Commands: The hero, angry that slavery still exists after the war where all of mankind joined against the dragons and reptiles, engages in a war against slavery. The masters are given especially violent executions...and one of the masters, an old underworld thug the hero was acquainted with, is beaten and given to her former slave-charges. The description of the rape is very brief, but very disturbing.