Even 150 years ago, Mormons had quite the reputation for both fighting for ( https://en.wikipedia.org/wiki/Mormon_Battalion ) and fighting against ( https://en.wikipedia.org/wiki/Utah_War ) the U.S. Army.
replies(2):
tldr: 120 emigrants were traveling by wagon train towards California, passing through Utah. They set up camp south of Salt Lake in a meadow for the night and woke up to being surrounded by the Mormon militia, painted as Native Americans who then opened fire on them, killing 120 men, women, and children. Their goal was to frame Native Americans of the massacre.
Mormons did spare any child under the age of 8 because Mormon Doctrine states that children under 8 are innocent and they were taken and raised into Mormonism.