Note: I am not the biggest fan of the moive September Dawn since it weaves a fictional tale about a Mormon family within the factual events of September 11, 1857. To watch a shortened version of the film with most of the fictional aspects edited out, see the youtube video here.
So how could a group of good and active Mormons kill innocent people? Well, the video [now posted below in the comments section] quoting LDS leader Dallin Oaks about blind obedience and how members must be basically loyal and not criticize their leaders (which is a tradition in Mormonism) no doubt contributed to the Mormon murders in 1857. See the 2007 lecture at Stanford on the The Lucifer Effect by Philip Zimbardo.