Most religions have had their fair share of atrocities like the Mountain Meadows Massacre. I don't think you can hold that against the Mormons, but it should be pointed out that the head of the Mormon church is considered a prophet and that all LDS followers, including Mitt Romney, must unquestioningly abide by his dictates as if it were the word of God.
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http://bible.cc/1_corinthians/13-11.htm