In the beginning of the film Once Upon a Time in the West (1968) the main bad guy, Frank, kills an entire family at a ranch including the youngest child of the family. The scene is meant to show how brutal and heartless Frank is as a villain.
Later, when Harmonica and Frank finally have their duel, we learn in a flashback sequence that
Frank killed Harmonica's brother when he was a child. In the scene, the brother is standing on a young Harmonica's shoulders with a noose around his neck as Harmonica struggles to help him balance. Frank sticks a Harmonica in his mouth to taunt him, and thus we learn the origin of his character. He ultimately is unable to keep his brother up and falls to the ground in exhaustion.
My question is, if Frank is a heartless villain who will kill a child so easily to keep from being named, then why did he let young Harmonica live?