Blood atonement

Blood atonement was once a teaching of the Mormon Church. Brigham Young first preached about blood atonement in the 1850s, though Joseph Smith may have thought of it first. 
Blood atonement taught that murder is so bad that the only way the murderer could pay for their sins was to be killed. This was sometimes called "blood-for-blood" punishment because it was used to punish murder. However, sometimes it was also used for non-violent crimes, like denying or leaving the church or marrying someone of a different race. 
The idea of blood atonement led to firing squads being allowed in Utah for many years. It also may have led to the , where Mormons killed some emigrants from Arkansas.
At one time, most Mormons believed in blood atonement. Today, only some fundamentalists do.