I watched it and it was good overall as theater, but it was actually less gruesome than the real Mountain Meadows massacre.
The show makes it seem like the attack was a blitz and over in an hour.
Best information is that while the Mormon militia did attack while pretending to be natives, and they did decide to kill everyone when the cover was blown, the wagon train was armed and set up a defensive perimeter.
A siege ensued for five days. The wagon train was running low on water and supplies and the Mormons sent in officers to negotiate a surrender. The wagon train accepted, and the armed men laid down their guns and went with the Mormon officers.
A short distance away from the wagon train the Mormons opened fire on all the men who had surrendered. Then they murdered everyone else, women, elders, children old enough that they were deemed a security risk for their ability to tell.
18 children were alive near the end of the hostilities, the oldest, a girl around 10-12 years old was then shot dead in front of the rest after the militia decided she was old enough to tell on them.
17 children under the age of 8 were the only survivors and were taken in by local Mormon families.
When the US army came back to investigate in 1859 they recovered many of the surviving children and transported them back to relatives in Arkansas. An Army officer noted that were in a state of terrible neglect, underfed, unclothed, covered in mud, and with obvious eye infections.
