The event that happened is referred to as the Mountain Meadows Massacre - a very sad chapter in Utah and Church history. It was a single event in southern Utah led by local church and community leaders. Much has been written - both inside and outside of the church about it. The consensus is that Brigham Young who was the prophet of the Church at the time did not know about it until after it happened and certainly did not advocate it. The church condemned it back then and does so today. It was very tragic and absolutely horrific. 
 
If you want to learn more about it - here is a link with documented sources that tell about it:Link

when I talked to my dad about this, he pointed out that these pioneers proclaimed themselves to be the ones that persecuted and undirectly killed many mormons, and they proclaimed that they would come back and do the same. now this does in no way justify the proceedings, but it does point out that the attack wasn't necessarily unprovoked, and was comitted out of fear probably more than hatred(that's obviously my own speculation). I also speculate that a large nmber of groups probably would have reacted similary, though again that does not justify thier actions, i just don't think you should condemn the mormon church because of it. and also church members are not perfect, and as this was not done in accordance with God's will, it should not reflect upon the church.

This event took place September 7-11, 1857.  
 
This event is the tragic result of many bad event and resulted in the murders of 100 - 140 innocent people. Those involved were all tried in courts of law, the leaders of this were acting outside of Church Channels and were excommunicated from the Church, tried in a civil court, found guilty, and executed.  
 
To give a little back story the Mormons arrived in Utah territory in 1847 after being chased out of Missouri and Illinois across the country. They had suffered many injustices and even were assaulted and members murdered and raped by evil people.  
 
They we to court, petitioned the State and Federal government to no avail, there were even an executive order in effect in Missouri till 1970's making it legal to kill Mormons on sight.  
 
After many years of persecution and terrible crimes committed against them they arrived in a barren desert and turned it in to a prosperous oasis in the Rocky Mountains!  
 
They set up the Utah Territory and lived in peace for 10 years. Then the persecution came again. The Federal Government banned the Church of Jesus Christ of Latter-day Saints and sent a Army corp to invade the Utah Territory and install a federal appointee as the new governor.  
 
The Mormon's without out anywhere else to go prepared to defend there homes and families from an invasion.  
 
The already great distrust of outsiders (still see in parts of Utah) combined with the pending Federal invasion of Utah spelled doom for a caravan of migrants heading for California.  
 
Local men fearing this was a party of spies attacked the caravan with help from the Native Americans. Over a course of three bloody days upwards of 140 men, women, and children were murdered.  
 
 
The Church denounced the attack and helped the victims of this vile act. The Church has never tried to hid or distance itself from this terrible event. They had no hand in as a Church but accept responcibility since it was Mormons who committed these acts.  
 
The descendent's of the victims and the Church maintain a joint memorial in Utah that commemorates the terrible lose of life and stands as a reminder to all that violence and intolerance is never the answer. 
 
 
 
Some try and paint this as the LDS Church's dirty little secret but there is nothing secret about it we acknowledge it happened, we aided the victims, punished the culprits, and raised a memorial to remind us of those troublesome times! 
 
Thank you for your question Sarah! 
 
You made the right choice in seeking out the Mormons to ask questions about our faith. Many get false information from poor sources that seek to distort things about our history and faith!

This was a mormon act, its okay to make poor choices and have members that are not good. Unfortunately it seems like the LDS Community has a hard time admitting when it has made mistakes.