What was the biggest "killer" of the sioux on the reservation?

On the reservation, the Sioux faced many different issues that contributed to the "killing" of their tribes. The biggest of these killers was health related. Due to low to no access to reasonable healthcare and support, many people died needlessly. The Sioux were also spiritually and emotionally killed in the sense that they had their culture ripped from them and degraded. The Battle of Wounded knee became the true ending of this tribal unit.

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Answer is mass marketing.