It largely had to do with slavery. This was around the time of the Civil War and most members were then from Northern States. They also associated with Native Americans, which made some local people suspicious of them. And, of course, any unusual religion is going to attract unwanted attention, which is why they eventually headed for wilderness.
The following is excerpts from a book called, "Our Heritage."A circular, sometimes referred to as the secret constitution, was passed around by the opposition to obtain the signatures of those willing to eliminate the �Mormon scourge.� These feelings of animosity culminated on 20 July 1833 when a mob, numbering some 400 men, met at the courthouse in Independence to coordinate their efforts. Written demands were placed before the leaders of the Church calling upon the Saints to leave Jackson County; to cease printing their newspaper, The Evening and the Morning Star; and to not allow any additional Church members to come into Jackson County. When the mob found that the Church�s leaders would not agree to these illegal requirements, they attacked the newspaper office, which was also the home of the editor, William W. Phelps. The attackers stole the printing press and demolished the building."
Tar and feathering, beatings and assasinations happened fairly often there in the short time they were in Missouri.
If you'd like to read more about what happened in Missouri, including a massecure in which men and boys were locked into a building a shot at until it was believed they were dead, you can visit this page:
http://library.lds.org/nxt/gateway.dll/C...f=templates.0 The law authorizing the murder of Latter-day Saints was recently found and removed in Missouri--it was mistakenly left in place.
In Illinois, the newspaper editor called for the murder of Joseph Smith--and got what he wanted. Families were forced to flee Nauvoo in the winter, without adequate food or supplies. Many, including little children, died as a result.The governor wrote that the plan backfired, since killing the prophet didn't end the church or disperse it.