As discussed under Alma 42:31, the 1830 signature (the 22nd) was proofed against 𝓞 rather than 𝓟 (although that signature was set from 𝓟). Both manuscripts originally read with the singular hand (“slain by their hand”), but the 1830 typesetter changed the singular to the plural. He probably expected the plural because of the plural possessive pronoun their. When Oliver Cowdery proofed the 1830 signature against 𝓞, he accepted the 1830 reading, the plural hands, and emended 𝓞 to agree by inserting the plural ending—and in pencil, an indication that the change probably occurred in the print shop. (For the use of pencil in the manuscripts, see the discussion under Alma 10:28.) In theory, either singular hand or plural hands will work here in Alma 43:44. Elsewhere in the text, there are examples of both singular and plural for the phrase “slain by the hand(s) of ”:
In fact, the plural instance in Alma 43:46, only a couple of verses away from the singular instance here in Alma 43:44, was probably the source for making the change in the 1830 edition from hand to hands. The critical text will restore in Alma 43:44 the singular hand, the original reading in both manuscripts.
Summary: Restore the original singular hand in Alma 43:44: “and many of the Nephites were slain by their hand”.