They wandered off the reservation when told not to do so, after a long series of Indian depredations that usually included torture, gang rape and the indiscriminate killing of women and children.
The soldiers at Wounded Knee also included elements of the 7th Cavalry who would not have been inclined to treat the Sioux lightly.
There are lots of photos of the torture inflicted on captured US Army soldiers. They were savages, and got what they deserved.
How charmingly 19th Century of you.
THEY we were the savages, not us. Primarily because we were clever enough to label our victims.