The correct answer is Colonel John Chivington.
The person who commanded the attacks at Sand Creek was Colonel John Chivington.
The night on November 28, 1864, Colonel Chivington left Fort Lyon and went directly to the camps of Cheyenne and Arapaho, located in Sand Creek. In the morning, he commanded his troops –about 700 soldiers- to the attacks at Sand Creek. It was a massacre. Two-thirds of the people killed were women and children.