Here in his editing for the 1837 edition, Joseph Smith emended the plural desires to the singular desire. As discussed under Mosiah 18:10 and 18:11, Joseph frequently altered the plural desires in his editing. Evidence elsewhere in the text shows that either singular desire or plural desires is theoretically possible; the critical text will in each case restore the earliest reading. Here in Alma 20:26, Oliver Cowdery himself initially wrote the noun in the singular, but almost immediately he inserted the plural s in the space that he originally left between desire and to (there is no change in the level of ink flow for this inserted s). Joseph crossed out this plural s in his later editing for the 1837 edition.
Summary: Restore the plural desires in Alma 20:26, the reading of the earliest textual sources (“Ammon had no desires to destroy him”).