What is the theme in Riders of the Purple Sage by Zane Grey?

In Riders of the Purple Sage Grey developed four themes, three of which have universal application and one that is peculiar to the American West. The first is that revenge can be justified, the second is that love between a man and a woman is one of the noblest functions of mankind, the third is that moral judgments of character must be relative, and the fourth is that the role of the gunman was a favorable element in the growth of the West.

The theme of revenge is introduced early in the novel, and throughout the story the reader learns the details so that when vengeance is carried out at the end he feels a sense of satisfaction.