The Intruder (1962)

When Roger Corman wrote his autobiography which was entitled "How I made hundreds of films in Hollywood and never lost a penny" (or something like that), he wasn't telling a truth since the above movie which he produced and directed lost a packet.  It is his only "message" film and has the young William Shatner ex StarTrek (before he became a fat bloated ham) playing a racist agitator moving from town to town in the South trying to stir up hatred during the early years of integration.  A black preacher is killed and a high school student is accused of rape and nearly lynched before the locals see him for what he is.  Corman obviously felt strongly about the subject matter, but he never again made a proselytising movie; however it still packs a wallop.