FWIW - I attended nursing school at Wm. Beaumont Army Medical Center in 1977. One of my patient's was a very elderly gentleman who told me his father was a physician who worked on a number of Indian reservations in the southwest. Evidently, my patient regularly accompanied his father while he worked, and formed good relationships with many of the people his father treated. This man told me that several of his friend's parents told him that Indians did not believe in completely wiping out their enemies. One reason was the possibility of "bad karma", or angering their Great Spirit for such wanton destruction. Another reason was that they understood that by leaving some survivors, their fame and reputation was greatly enhanced by tales by the survivors.
He said that the Indians said they were shocked to see many of the soldiers shoot each other, then themselves. In other words, murder/suicide. I've read several books that reported that soldiers, fearing torture if captured by the Indians, made murder/suicide pacts with their friends.
I always thought what my patient told me made sense, and was an interesting insight into a possible facet of the battle.
Regards,
Dave