|Santa Fe Trail (1940) [John Brown]: Executed by hanging. (I haven't seen this movie, but I'm reasonably familiar with the history.)
Reap the Wild Wild (1942) [King Cutler]: Shot to death by Lynne Overman after Raymond shoots Robert Preston.
Mourning Becomes Electra (1947) [Brigadier General Ezra Mannon]: Poisoned by his wife (Katina Paxinou); he dies shortly after his daughter (Rosalind Russell) hears the noise and enters the bedroom.
Seven Angry Men (God's Angry Men) (1955) [John Brown]: Executed by hanging. (I haven't seen this movie, but again, I'm familiar with the history.) (Thanks to Stephen)
Ford Star Jubilee: The Day Lincoln Was Shot (1956) [Abraham Lincoln]: Shot in the head by Jack Lemmon in the theatre. (I haven't seen this myself, but we're all familiar with the history.)
Mackenna's Gold (1969) [The Preacher]: Hit in the back with a tomahawk by an Apache warrior.
Night Gallery: Clean Kills and Other Trophies (1971) [Colonel Archie Dittman]: Decapitated (off-screen) by supernatural forces, after Herbert Jefferson Jr. prays to his gods to put Raymond in his quarry's place; his head is shown mounted on the wall afterwards.