Movie almost no one has seen called 'The Lost Son'
Essentially, this private detective discovers then bluffs himself inside a building that has been turned into a hotel of sorts where sick fucks can rent children for sex.
In the scene, the detective is in the bedroom where a young boy - maybe 8 - is dressed in dingy t-shirt and underwear. He doesn't react to detective entrance, just stands meek, head down. Detective is freaking out about what he saw in anteroom (various profoundly disturbing sex and bondage devices) and assures boy, not here to hurt him. The detective starts looking around the room, talking to boy (whom you can not see and isn't saying anything).
When the detective turns back around, the boy is on his hands and knees on the bed.
I've seen A LOT of movies (over 2,000 including the movies everyone cites as worst - Audition, Salo, Serbian Tale, Cannibal Holocaust etc) and nothing has bothered me as much as that moment.
When the detective kills the guard outside the room, I raged at the screen 'MAKE IT HURT. MAKE HIM SUFFER'