As to these guys motive for murder.
Well Brigham Young laid down the law regarding "
blood atonement" and the "Sons of perdition" in September 1856 in the "Journal of Discourses".
...."I do know that there are sins commited, of such a nature that if the people did understand the nature of Salvation, they would tremble because of their situation.If they knew themselves of the only condition under which they can obtain forgiveness, would beg of their brethren to shed their blood, that the smoke therof might ascend to God as an offering to appease the wrath that is kindled against them"....This woman had committed the unforgiveable sin,(turning aside from the truth) and only her death could cancel out her sins so as she could be back in Gods grace. There you have it in a nutshell.