The Day of the Gun (A Breed Western #17). Breed, #17

Matthew Gunn, alias Azul-half-white, half-Apache, all-killer-was just passing through Valverde, a two-bit town down on the border. So was Honest Abe Cannon, an ageing shootist with a deadly reputation. Abe Cannon, weary of fighting and killing, was off to claim his ranch in Utah and spend the rest of his days in peace. The shoot-out in Valverde proved that Abe's days as a top gunfighter were numbered, So he hired Azul to escort him and his saddle bags full of money through hostile Apache country.
But on their trek north, Cannon and Azul ran into the savagery of Indian justice-a justice which Azul understood and meted out, but which Cannon would have to fight to understand.