Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
A gang of outlaws kill a US Marshal because he put them in prison years before, then they plan to sell rifles to a renegade band of Apaches. One of the outlaws decides he's had enough and rides off, but they convince the Apaches to take care of him and his woman the next day. Sterling Hayden, the ex-outlaw now Deputy Marshal who's chasing them 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 he is slowly pulled towards a rattlesnake that is also tied to a stake not far from his head. The Marshal takes the torturous opportunity to interrogate him 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 no Humane Society monitoring of the animal action in this scene: they used a real Northern Mohave Rattlesnake that was actually attached to the 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.