If this seems too tawdry for a 1956 film, consider this:
According to John Wayne in a 1974 interview, John Ford hinted throughout the movie that Ethan had had an affair with his brother's wife, and was possibly the father of Lucy and Debbie. This meant Ethan's thirst for vengeance stemmed not from the murder of his brother, but of the woman Ethan had loved. This was so subtle that many viewers at the time missed it altogether.There is a constant mention of the significance of "blood kin" made by Ethan and his comparison of Marty vs. Debbie's relationship to him. Despite his strong feelings on the subject, he makes a point to draw up a his will and leave it to Marty (still stating they have no blood relationship).
-- [from http://www.imdb.com/title/
Still, the reaction to Marty's mother's scalp must be telling. I'm just not sure why I've never heard this before. The Wikipedia entry for this film says "The Searchers is one of those rare films that reveals something new with every viewing". How true. I've seen this film probably a dozen times and never thought this before. Amazing film!