The idea of imperial expansion and colonization helps elucidate the Mormon horns phenomenon.  Similar to Yankee horns and Catholic/Protestant horns, Mormon horns seem to occur mostly in the context of “invaders”/local minorities communicating with the center/majority. As in those cases, They-believe-we-have-horns (TBWHH)
(1) Demarcates Us and Them
(2) Paints the Us as victims and
(3) Paints the Them as stupid / uninformed / unenlightened
Allegations of horned-ness work when people actually believe some humanoids have horns and when everyone agrees that there are no such things.  Since nineteenth-century “progress” so self-consciously associated the medieval devil with backwardness, to allege belief in literal horns constructed that person as emphatically Other. The meaning and connotation probably has not changed that much up to the present. At any rate, (2) justifies the Us’s sins against the Them because They were mean to Us and (3) justifies the Us’s colonization of Them. In the Mormon case, (3) received particular attention in the endowment ceremony wherein Protestant ministers were portrayed as deceived by Satan. 
In a different vein, and as illustrated by Yankee horns, especially decades after the war, horns
(4) Bandage the past and allow for reconciliation by providing
(a) An ignorance out—if we had known… we wouldn’t have had the war… and
(b) A relatively neutral usable past with follies to chuckle over while avoiding the pillage, rape, murder, discrimination, etc. 
Of course, the loser gets incorporated into the winner’s narrative. Yankees tell stories about Southerners and not vice versa. Mormon propagation of TBWHH unconsciously, then, seizes the winner/conqueror narrative. Another thing TBWHH does:
(5) Enhances the prestige of individual teller: missions to dangerous/exotic places are “cooler” in some social contexts than others.  It can also
(6) Account for lack of success: if the people are so misinformed that they think you’re not human, of course they’re not going to let you baptize them.
A usage from 1870 inverts the failure application. A writer identified as “A Cynic” argued:
There is no proposition, however ludicrous, for which you cannot obtain proselytes…. The sight of a real live Mormon first persuades ignorant people that Mormons have not necessarily horns and hoofs, and from that it is only one step to a belief in the mission of Brigham Young. 
Actually, “A Cynic” does construct him- or her-self as a minority, but as an intellectual rather than sectarian or sectional isolate: the locals are so dumb that all you have to do is show them that Mormons don’t have horns and there they go. Mormons used the story to explain the failure of local church efforts or the miraculousness of their success while “A Cynic” used exactly the same story to explain the facility of Mormon success. 
Mormon Horns 1, 2, 3, 4, 5, 6, 7. These horn posts continue earlier efforts analyzing ways of describing Mormons or using Mormons to describe something else. These include: India, Cows, Bluebeard, Lice, Crickets,Flies, Happy Valley, and sundry other beasts.
 However, we should be clear that we’re talking about The Middle Ground type of colonial interaction rather than a stimulus-response exchange between two discrete entities (with all their pre-existing complexities). That is, the “middle ground” is a shared geographic and imaginative space that is equally new—or made new by the encounter with the Other—to both entities and in which both (again with their various complexities) must re-imagine, recreate, and redefine themselves, which reformulation often involves changing which physical bodies and physical spaces count as “us” and “homeland.”
 The first allegation impugns the people with horns (see medieval depictions of horned Jews); the second impugns those who allegedly believe such “silly” stories.
 Since TBWHH appears in so many other contexts, I think it is a mistake to emphasize the influence of the temple in the horn question, but surely it didn’t hurt. “Two other features dropped were a dramatization suggesting that Satan beguiles Christian clergy to teach false doctrine and the requirement that members make throat-slitting and disemboweling gestures as signs that they will not reveal the ceremony’s contents.” John Dart, “Mormons Modify Temple Rites – Ceremony: Woman’s vow to obey husband is dropped. Changes are called most significant since 1978,” Los Angeles Times, 1990 May 05.
 It transforms fratricide into the “joy of fratricide.” See Benedict Anderson, Imagined Communities. The idea is someone else’s, but he develops it in the identity/colonial context.
 I certainly get more social mileage about the one time I think I ate cat (identity not reliably confirmed) on mission in Brazil than from the months I spent working in Georgia before my visa came through.
 [Part of the ellipsis]: “All oratory comes simply to this: here am I, a visible and tangible human being, who hold such and such a doctrine, and am more or less affected by it. Other human beings catch the belief as they might catch the scarlet-fever, not by an intellectual process so much as a spontaneous sympathy.” [After the quoted portion]: “…as long as men have other faculties than those necessary for multiplication and division, orators will have a field in which competition is, and must be, out of the question.” “ A Cynic, “Oratory,” The Cornhill Magazine 22:127 (London: 1870 Jul): 89 [87-97].
 Joseph F Berg made a similar argument eighteen years earlier. “[I]t appears that a New Hampshire man is…labouring to prove that the world shall surely be destroyed in 1854, and the probability is, if Joe Smith could gain over half a million of converts, …and after his death leave Brigham Youngs enough to carry on his imposture, this new prophet will, as the Yankees say, make a good thing of it. … If you will look into the haunts of infidelity, or …the ranks of those…captivated by any novelty, …you will find the material out of which impostors can manufacture a large amount of capital…. It matters not what the thing is…. They will speculate in spirit rappings, or in socialism, in Mormonism, or in Millerism…. ¶ … On this principle we can account, at least partially, for …the popular mania respecting witches, two centuries ago. … One of the main sources of this delusion is to be found in the legends of the dark ages. … The popular notion of the Evil One represented him as a large ill-formed, hairy sprite, with horns, a long tail, cloven feet, and dragon’s wings. …These and similar stories… paved the way for scenes of terrible tragedy. …” Joseph F Berg [signed “Editor”], “Moral Epidemics,” The Protestant Quarterly Review 10:2 (Philadelphia: 1853 Apr): 106 [97-108].