I can't say you're wrong in your assessment, but just a little bit of comment. 1906 wasn't long after the Indian Wars and a very large precentage of the American people was very prejudiced against Indians because they thought the Indians were blood-thirsty and that the whites had a right to eliminate them because they were not civilized (and that was a lot of BULL). Many whites considered Indians (and blacks) as being of a lower order of being, no better than than animals and not worth letting live. They called Native Americans Injuns and other other none whites more derogetory terms. During World II, this attitude grew about orientals Calling the Japanese, Japs or Nips and the Germans were called Krauts and again there were a lot of more derogetory terms.
Having said all that, I ask you to remember...Zane Grey was a product of a certain era and the morality or lack thereof of his characters was likely reflective of the people he wanted to have buy his books. I know it's not an excuse, but a writer of today possibly wouldn't be published for thins like this.
Read some of the war comics and others on this site and you'll probably see that I mean. Remember in reading period material, it is important to know the "culture" and prejudice of the time. Try top understand the way people thought and talked in other eras.
Thank You for your time