This would explain why Jack ended up with a bounty on his head; he killed Natives who were working for the government. It would also explain why he shows no prejudice against Red Harvest. He doesn't hate Native Americans in general, just a specific tribe.
WMG / The Magnificent Seven (2016)
Jack Horne's wife and child were Native Americans.According to the Pigeon Brothers, Jack made a name for himself by killing Crow. Historically, the Crow Tribe were allies of the US government and worked with them against their enemy tribes such as the Sioux and Cheyenne. Perhaps Jack's wife was from one of those tribes.
Billy Rocks was originally a railroad worker.A large number of Asians in America during this part of history were railroad workers. He also shows some expertise with dynamite. Goodnight says he met Billy while serving a warrant from the Union Pacific Railway, implying that Billy and the railroad had a history together and didn't part on good terms.