The title is based on the biblical story of the curse of Cain and the mark of Cain, where God declared that Cain, the firstborn son of Adam and Eve, was cursed for murdering his brother, and placed a mark upon him to warn others that killing Cain would provoke the vengeance of God.
When Sadler was shot in the marshal's office, it seemed he died but at the end he rides out of town with the dead buffalo hunter singing and nobody was noticing. Departing spirits.