The 1874 RLDS edition introduced two changes here. First, the therefore at the beginning of the verse was omitted, perhaps unintentionally. Second, the verb phrase had become was replaced with had been. Although probably accidental, this innovative reading (replacing the dynamic become with the static be verb) implies an inherently sinful nature: “as they had been carnal sensual and devilish by nature”. To the contrary, Alma is arguing here in Alma 42 that man became carnal, sensual, and devilish by nature as a result of the fall (when man partook of the forbidden fruit). The incorrect been has been retained up through the current RLDS edition (1953), even though the 1908 RLDS edition was in general revised according to the reading of the printer’s manuscript.
Summary: Maintain in Alma 42:10 the sentence-initial therefore and the use of become rather than been; both therefore and become are extant in 𝓞; moreover, become describes much more accurately Alma’s argument that only as a result of the fall did man become carnal, sensual, and devilish by nature.