Stanley Kubrick’s A Clockwork Orange
https://en.wikipedia.org/wiki/A_Clockwork_Orange_(film)
The violence is portrayed in such an off hand, random and casual way, I found it very, very difficult to watch. Even Kubrick requested it’s withdrawal from cinemas in the UK. When I saw it on television about ten years ago, it was shown late at night and came with a strong warning / caveat. For me, no “horror” comes within a country mile, of just how disturbing some of the scenes in this film, present violence.