Your description of that action and what I have heard differ greatly. My understanding is that the Mormons got ready to burn the city and allowed the US Army to enter the city with no one being hurt on either side.
They worked out an agreement with the US Army and after a while the army left because they considered the Mormons peaceful and there was no need to stay.
The Mountain Meadow Massacre was a horrible event. While there have been many people who accused Brigham Young of ordering this action there has never been any proof, in fact the time line of the event suggest an impossibility of that being the case. There is more to the story. Out west water is the most precious commodity there is. The wagon train that was massacred was accused of poisoning every well they used when they left. This means death to the people who use the wells. While there are those that say that this particular wagon train was not responsible for poisoning all the wells there is evidence that they were. We will likely never know.
It is hard for me to understand how any person in the name of religion can kill another, or for that matter poison their wells that they themselves were using. My mind will just not go certain places.
Despite what we may think of Mormon theology the ones I have met, and I have met many, have all been very nice charitable people. It is totally unfair to cite the Mountain Meadow Massacre without mentioning the atrocities heaped upon the Mormons as they were run out of cities they built, as they and their children were murdered. It was only a few years ago that the governor of Missouri lifted the extermination order put in place when the Mormons lived in that state and in Illinois.
When you deal with the Missouri War and the Illinois War, you'll find there is more than enough blame to go around, as well as more than enough exaggerations.
Of all of the stories I've ever read about the MMM, this is perhaps the most ridiculous I've ever read. Source?
Take care, my FRiend.