Here the 1841 British edition and the 1874 RLDS edition independently, it would appear, removed the repeated a before murderous disposition. This change appears to be accidental. Typically in English, we do not repeat the indefinite article in conjuncts of nouns, but this kind of repetition is characteristic of the Book of Mormon. Elsewhere in the original text, where wicked is conjoined with another adjective, the indefinite article is always repeated:
For a general discussion, see under conjunctive repetition in volume 3. The critical text will restore the repeated a here in Alma 43:6.
Summary: Restore in Alma 43:6 the repeated indefinite article a in “a more wicked and a murderous disposition”.