The typesetter for the 1841 British edition accidentally replaced should with would here in Alma 42:19. He was probably influenced by the preceding would in “would he be afraid”. The original manuscript is not extant here, but the printer’s manuscript reads should, as do the first three editions (1830, 1837, and 1840). Further note that in the original text the later “he should die” is identical with the earlier “he should die” that occurs just prior to “would he be afraid”. There are numerous passages elsewhere in the text where would occurs in the main clause and should in the dependent clause, as in 1 Nephi 10:6: “wherefore all mankind was in a lost and in a fallen state and ever would be / save they should rely on this Redeemer”. The critical text will restore the original modal should here in Alma 42:19 (“would he be afraid he should die if he should murder”).
Summary: Restore in Alma 42:19 the original should in “would he be afraid he should die if he should murder”; the current LDS reading, “would he be afraid he would die if he should murder”, derives from an 1841 typo influenced by the preceding occurrence of would.