It was Brigham Young who was a driving force behind the Mountain Meadow Massacre. Like Joseph Smith, his name is synonymous with the Latter Day Saints. Like all men, including those God chooses to use, these men had their faults as do I.
You're right about one thing: The Latter-day Saints have never claimed that any of our prophets have been perfect. Brigham Young clearly had his faults. Sanctioning the Mountain Meadows Massacre, however, was not one of them.
From the Encyclopedia of Mormonism:
Here is the story of the tragic events which transpired in the summer of 1857: The Mormon pioneers, having been forceably driven from their homes in Illinois in mid-February of 1846, had in 1847, finally arrived in the Salt Lake Valley, where they had come to worship God without the persecution they had endured since the Church was founded in 1830. They were, at this time, working at colonizing the West. In 1857, Utah was not a state but a territory. As the events unfolded, a large contingent of US troops was marching towards the Utah Territory. Despite having been the federally appointed territorial governor, Brigham Young was not informed by Washington of the army's purpose, and interpreted the move as another surge of persecution of the Latter-day Saints. Anticipating an imminent attack, he declared the territory to be under martial law and ordered the Saints to stand ready to defend themselves.
Part of Brigham's strategy was to enlist the local Indian tribes as allies. He had not long before made a statement urging the settlers to try to gain the confidence and trust of the Indians, saying, "they must learn that they have either got to help us or the United States will kill us both."
Meanwhile, a group of non-LDS emigrants were headed for California. Due to the lateness of the season, they had decided to take a southern route, which would take them through the town of Cedar City and thirty-five miles beyond it to Mountain Meadows. This area was fairly well-known as a good place for travelers to stop and rejuvenate themselves before starting off into the vast desert that lay between the Utah Territory and the Pacific Coast. This particular party was made up of a diverse group of about 120 people, most of whom were from Arkansas.
As the emigrants made their way from Salt Lake City towards Cedar City, tensions arose between them and both the Mormons and the Indians. Spurred by rumors, their own observations and memories of the horrible attrocities which had been perpetrated against them in the past, Mormon residents in and around Cedar City were anxious to take action against the travelers -- even though the travelers themselves were innocent of any wrong-doing against the Mormons. Ultimately, however, they decided to hold off until they were able to contact Brigham Young and ask for his blessing in doing so. They dispatched a messenger on September 7 to make the nearly 300-mile (one-way) trip to Salt Lake. He made the trip in just over three days.
Within about an hour after meeting with Brigham Young, the messenger was back on his way to Cedar City with a letter from Brigham Young with the following statement:
"In regard to the emigration trains passing through our settlements, we must not interfere with them until they are first notified to keep away. You must not meddle with them. The Indians we expect will do as they please, but you should try and preserve good feelings with them."
The messenger arrived back in Cedar City on September 13. By that time, nearly all of the men, women and children who had been on their way to California had been slaughtered by the Mormons and the Indians. (About eighteen children were spared.) It was a tragic, senseless act of violence commited by a group of people who undoubtedly knew better but who had, for whatever reasons, let their desire for revenge override their common sense and religious training. As more information became available, several of the principle participants were formally excommunicated from the Church.
