Some Western MO residents...
...were ungodly and violent (Missouri had its share of hoodlums & roughians like all other states…perhaps even more than their share given the pro-slavery activities they went out of their way to do in Kansas in the 1850s…)
...didn't like it that the Mormons contributed little to the local economy as they tended to transact only with Lds businesses.
...resented potential loss of political control.
But also, Missouri residents kept hearing Joseph Smith & local Mormons talk about how Jackson Co. would become the world HQ for the Mormon church and how it would be run by its own sets of laws pertaining to the "celestial kingdom" ... For example: "And Zion cannot be built up unless it is by the principles of the law of the celestial kingdom; otherwise I cannot receive her unto myself" (D&C 105:5, June 1834)
At that time, “Zion” = MO!!!
Consider this evidence -- testimony from 19th-century MO Judge Joseph Thorp: Their prophet also induced his followers to believe that he would form a temporal kingdom or government, and they would not be subject to the laws of the State, but should make their own laws, have their own civil officers to execute them, Joseph, the prophet, being dictator, aided by revelation and his cabinet or council; and when their edicts were sent forth they were obeyed without a murmur by his followers. Judge Joseph Thorp, Liberty Tribune, "Early Days in Missouri," October 12, 1883. No. 23.
Beyond the reasons above, some feared a physical takeover. Judge Thorp again: Their audacity and impudence in telling the citizens that it was made known to Joseph, their prophet, priest and king, from high heaven, that Jackson county was theirs -- given to them by the Lord, and it was foolishness in them to resist and fight against God; that the temple was to be built in Independence and that saints were to be gathered from the four quarters of the globe to worship the God of Israel in the New Jerusalem, as it was under the Jewish dispensation. Their idea was, it appears, that they were to establish a spiritual kingdom where the latter day saints were to congregate to worship tho God of Ancient Israel as did their fathers in the days of the prophets; they commenced preparing to build the temple, and, if I recollect, partly dug out the foundation. (same source)
(Judge Thorp also added that Their paper was filled up weekly with revelations, promising great things to the saints who were faithful, and threatening destruction to the citizens if they did not give up their lands and homes peaceably, and leave them in peaceable possession, contending that the Jew and Gentile could not live together in the same locality. Certainly, Smith put these threats into Lds "scripture" by June of 1834 that the ungodly would be hit with vengeance like a whirlwind (v. 22), god-based scourging & vexing (v. 23), the indignation of the lord (v. 24) in D&C 105. Also, as I mentioned, many were pro-slavery & identified with Southerners, whereas Lds tended to be Northerners/Easterners & were perceived as being "abolitionist" even though Smith himself uttered a D&C "scripture" where he said the Mormon gospel was off-limits to slaves minus their "master's" consent.
...And then many didn't like Independence becoming the world-wide HQ military takeover Smith was preaching as early as June of 1834: "And after these lands are purchased, I will hold the armies of Israel guiltless in taking possession of their own lands, which they have previously purchased with their own moneys, and of throwing down the towers of mine enemies that may be upon them, and scattering their watchmen, and avenging me of mine enemies unto the third and fourth generation of them that hate me. But first let my army become very great, and let it be sanctified before me, that it may become fair as the sun, and clear as the moon, and that her banners may be terrible unto all nations; That the kingdoms of this world may be constrained to acknowledge that the kingdom of Zion is in very deed the kingdom of our God and his Christ; therefore, let us become subject unto her laws." (D&C 105: 30-32)
Smith was even proclaiming that all nations would bow down to the Lds church in a new "scripture" established on Aug. 2, 1833: "...if Zion doing these things [build a house of the Mormon god & school + obey commandments] she shall prosper, and spread herself and become very glorious, very great, and very terrible. And the nations of the earth shall honor her, and shall say: Surely Zion is the city of our God, and surely Zion cannot fall, neither be moved out of her place... (D&C 97:18-19)
Consider this evidence -- testimony from 19th-century MO Judge Joseph Thorp:
Their prophet also induced his followers to believe that he would form a temporal kingdom or government, and they would not be subject to the laws of the State, but should make their own laws, have their own civil officers to execute them, Joseph, the prophet, being dictator, aided by revelation and his cabinet or council; and when their edicts were sent forth they were obeyed without a murmur by his followers.
Judge Joseph Thorp, Liberty Tribune, "Early Days in Missouri," October 12, 1883. No. 23.