This is the tragic and real story of what happened at the Mountain Meadows Massacre in 1857, where a Mormon militia (Latter-day Saints) attacked a wagon train and ended over 100 people, leaving only 17 small children alive. This event is highly fictionalized in the Netflix show American Primeval, but here is what actually happened.
Why Were Violent Acts like the Mountain Meadows Massacre Committed by Latter-day Saints?
https://scripturecentral.org/knowhy/why-were-violent-acts-like-the-mountain-meadows-massacre-committed-by-latter-day-saints