The Sadist
(1963)

Ed, Doris and Carl are teachers driving from a small town in California to LA for a Dodgers game. They have car trouble and pull over into a remote junkyard and repair shop. They are taken prisoner by Charlie, a homicidal maniac on a cross-country killing spree with his girlfriend Judy.  Charlie torments them and clearly likes watching them suffer.  Charlie shoots Carl, but needs Ed alive to fix the car so he can get out of there. Charlie hints to Doris that he will assault her when both men are dead.

While working on the car Ed sprays gas in Charlie's eyes. Charlie shoots at the blurry figures and kills Judy by mistake.  Doris runs off into the desert to escape. Charlie pursues Ed through the junkyard. When Charlie's gun runs out of bullets, Ed goes to jump him. However, Charlie is able to reload too fast and shoots and kills Ed.

Charlie heads into the desert after Doris. Charlie falls into a hole with rattlesnakes and dies from the snakebites.  Doris is left alone in the desert, the only one alive.

Thanks Joseph C!