The simple fact is that the settlers were killed in cold blood: men, women and children down to the age of 6. Such things do happen in war, and the Mormons were, so to speak, at war with the United States. They had fled the United States for the sanctuary of Mexican Territory, Then, all of a sudden , they were back in it.
Mormon exodus and Mexican War, 1846.
MMM - 1857.
Your account makes it sound like the two events were closely linked in time.
A greater factor was the strife between Brigham Young and the US government at the time, which many on both sides expected to result in all-out war, but which never quite reached that stage.