FAIRLDS.org, one of my favorite Mormon information sites, has a helpful review of Jon Krakauer's Under the Banner of Heaven: A Story of Violent Faith.
I've also compiled some information on the Mormon Danites.
Some critics and misinformed novelists have painted the early Church and Salt Lake City as a scary place, where murder was rampant and non-Mormons lived in fear. That just doesn't make any sense to me. Non-Mormons were treated well and co-existed peacefully. Their right to worship was protected, in Salt Lake and other Mormon cities like St. George. One of my ancestors, Luther Peet Lyon, who practiced law in New York state and then went to California looking for gold during the gold rush era, stopped in Salt Lake City on his was back home. If this was such a hostile and frightening place for non-Mormons, it's amazing that he was able to safely meet with Brigham Young, walk safely through the streets, and like what he saw enough to stay and be baptized in 1865.
There have been violent Mormons, of course, as there have been violent people of all faiths. But it is usually wrong to blame the religion for the abuses of a few evil people.