How were members of this religion treated in Illinois?

The hostility towards the Mormons was very high in Illinois. This anger and violence even resulted in the assassination of Joseph Smith, the Mormon leader, that was performed by the angry mob.

This whole conflict continued long after the assassination of Joseph Smith (1844) which is called the "Mormon War in Illinois". In Illinois "wolf hunt" was started, but the "wolves" were codes for Mormons. They were met with intolerance and anger in all the aspects.

The hostility continued and finally in 1846. it resulted in the forced expulsion of all the Mormons from Nauvoo, Illinois.