I read up on the Mountain Meadow massacre and it is very complicated. The group killed off were against the Mormons. They wanted to get rid of all Mormons. When they were killed off, only one man was executed for the crime.
Actually, it's not complicated at all. The Francher Party was on their way to California. The Mormons first instigated the Paiute Indians to attack them, and when that failed, they promised them safe passage, and slaughtered all of them except a handful of small children.