Sand Creek was the Colorado regiments being glory hounds looking to prove themselves, since they were just guarding gold fields, miners, and transportation links instead of fighting the Confederates outside a few units at Gloreita Pass. It was a massacre brought on by ignoring reports, wanting to 'blood' units, and general indian hate.*clears throat* Oh, really?
White folks weren't always the nicest people either, you know. Maybe look at what happened at Sand Creek, for example.
The people killed at Sand Creek were pretty much all innocent.
However, there is a reason Cortez was able to get help from the local tribes taking down the Aztecs, and why groups like the Commanche were hated by their neighbors long before the Spanish showed up.