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.
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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.
Years ago in Arizona I worked with a guy who was a descendant of John D. Lee who was the ringleader and later executed for his role in this attack. Lee's Ferry over the Colorado river is named for him also. So I looked into John D Lee a bit.
Here is his account:
http://law2.umkc.edu/faculty/projects/ftrials/mountainmeadow...
The people in the wagon train were believed to have been those who had burned the Mormons out in Missouri, and although it seems revenge is a more likely motive then fear, there may have been an element of both. At any rate it wasn't just bushwhacking random wagon trains as many passed through the territory at the time.
I found Lee's account an interesting read and his grandson (or great grandson) looked very much like him.