Joseph Smith's death at the hands of a mob at Carthage, Illinois, spurred a Mormon exodus from Nauvoo. It was during this time of upheaval that Rockwell shot and killed Frank A. Worrell, who was menacing Hancock County Sheriff Jacob Backenstos. Rockwell had been hastily deputized only moments before the shooting, a fact which made the incident no less sensational when it was learned that the dead man had been the militia lieutenant in charge of protecting Joseph Smith when the Mormon prophet was assassinated the year before.

The Mormons, now under the leadership of Brigham Young, crossed the plains to Great Salt Lake Valley in 1847. Rockwell was one of the territory's earliest lawmen--deputy marshal for the provisional state of Deseret in 1849. When President James Buchanan appointed Alfred Cumming to replace Brigham Young as Utah's governor in 1857 and ordered a large contingent of U.S. troops to escort the new chief executive to his mountain offices, Rockwell was among the number of Mormons chosen by Brigham Young to harry and harass that "Utah Expedition," which Young considered nothing less than an invasion "by a hostile force who are evidently assailing us to accomplish our overthrow and destruction."