Which quotation form "A Horseman in the Sky" shows how Carter Druze's father advances to the plot of the story?

From all the phrases of the story, "Whatever may occur, do what you concieve to be your duty", is the most impactful quote to the plot. Not only is it mentioned twice, but it has a signficant relation with the story's climax.

This quote is what determines Carter's behaviour throughout his encounter with the horseman, but it only takes full effect once the horseman faces his direction, revealing his identity to Carter, but not to the reader. At the end of the story, it is revealed that the horseman is in fact Carter Druse's father, and so it is proven that both Carter and his father were in opposite sides of a war.

This revelation causes Carter to shoot the horse of his father, implying he didn't have the nerve to kill his own father, but at the same time, attempting to live to his father's counsel: "Do what you concieve to be your duty".