Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
In this old Western, a gang of outlaws kill a U.S. Marshal because he put them in prison years before. Then they plan to sell rifles to a renegade band of Apaches. One of them decides he's tired of being an outlaw and rides away. The others convince the Apaches to get rid of him and his woman the next day.
Sterling Hayden, an ex-outlaw now Deputy Marshal who's chasing the gang for personal revenge, rides up to the rattlesnake scene about 21 minutes into the movie. He finds Yvonne DeCarlo tied to a tree next to the outlaw who is lying on his back with his feet tied together. His hands are spread-eagled and tied with rawhide to two stakes in the ground. As the rawhide tightens up and gets shorter by the sun, he is slowly pulled towards a rattlesnake that is also tied to a stake not far from his head. (This is supposed to be how Apaches tortured people, but I wonder if it's completely made up.) The Marshal makes use of the opportunity to interrogate the outlaw as he inches toward the snake. Once the Marshal learns what he wants to know, he cuts the man loose and shoots the snake with his handgun, blowing its head off.
There was obviously no Humane Society monitoring of the animal action in this scene: they used a real Northern Mohave Rattlesnake that was actually tied to a stake - you can see it struggling to get free - and they killed it. It's head was shot off or exploded with a small charge.