Blood atonement is the Mormon teaching—rarely discussed or believed today among practicing Latter-day Saints—that some sins require the shedding of one's own blood to be forgiven by God. It ties in with symbolic penalties that were a part of the LDS temple ceremony (these penalties were removed in 1990), where participants covenanted to be subjected to violence if they betrayed LDS teachings. The Mormon Reformation was a specific period in Mormon history, 1857–58, when church leaders such as Brigham Young, Heber C. Kimball, and Jedediah M. Grant, emphasized a return to strict LDS beliefs and practices.

The line between "blood atonement" and western frontier violence is hazy and thin. In other words, there was certainly violence in the Mormon colonies in Utah Territory and the West, but how much of it was caused by the specific doctrine of blood atonement is debatable. In fact, I think it's likely that little to no violence occurred just because someone needed to be killed in order to be saved. Rather, the violence was usually clashes between Native Americans and Mormon settlers. Other violence was over more traditional disputes, such as land, jealousy, robbery, water rights, etc. Mormon leader Parley P. Pratt was killed by Hector McLean, for example, because Pratt had taken McLean's estranged wife as one of his own plural wives.

Mormon leaders did preach violence and did suggest some people needed to die. In a meeting of the Council of Fifty, leaders said Ira West would need to lose his head. But such rhetoric was rarely carried out. So while dime novel literature at the time (most famously the first Sherlock Holmes story, "A Study in Scarlet") portrayed Utah as a dangerous place for non-Mormons or ex-Mormons, the reality was different. There were always Mormon dissidents in the territory, ex-Mormons willing to criticize church leadership, and even powerful anti-Mormon movements that worked hard to elect non-Mormons to public office. The Salt Lake Tribune, founded in 1870, began as an anti-Mormon newspaper and was critical of the church.

The obvious violent act of the reformation era was the Mountain Meadows Massacre of September 11, 1857. Mormons, with the aid of Paiutes, murdered at least 120 men, women, and children in the Baker–Fancher train from Arkansas. This came about because of the hysteria of the Mormon Reformation and false rumors that reached Mormons that the wagon train was dangerous. Mormons also refused to sell goods to the travelers, something that exacerbated tensions. While a handful of historians believe Brigham Young himself ordered the massacre, there's no direct evidence for that and most historians believe Young was not involved. However, his violent rhetoric contributed to the environment that caused the massacre, and he was a part of the cover up that occurred after the massacre.

Book exposes, such as "Wife No. 19," "Brigham's Destroying Angel," "Rocky Mountain Saints," "Tell It All," and others, have long been tricky for historians to sort out. On the one hand, these books were written to sell copies and they include lurid details that have no basis in reality. On the other hand, stories from former Mormons, especially those who were close to church leaders like Young, offer valuable insights and cannot be entirely dismissed outright. Historians of Utah and Mormonism have learned which books are more reliable than others.

For good summaries of this, see David Bigler, "Forgotten Kingdom," and Brooks, "Mountain Meadows Massacre." For alternate views on the massacre that assign more blame to Young, see Bagley, "Blood of the Prophets." John Turner, "Brigham Young," also touches on themes of violence.