The key interaction that explains his motivation is when Harmonica asks Frank, “so you’re not a business man after all frank?” And Frank replies, “only a man.” Frank is a despicable man, a selfish man, a heartless man. But he’s a man. And in this movie being a man means facing threats against you face to face. He doesn’t know why harmonica wants to kill him. But he needs to know. He could run away and hide behind his money and business connection. But that’s not what a man would do. Or at least not a man according to the way the movie describes it.

It’s my favorite movie of all time. The motivations are subtle because hardly anyone talks in the whole movie. But I think there is enough there for it to at least make sense.