The 1858 Wright edition accidentally replaced the infinitival to with the preposition of, thus giving “afraid of sin” instead of the correct “afraid to sin”. Given the context, the to clearly works better. The 1874 RLDS edition ignored this typo in the 1858 Wright edition and set the correct to of the earlier 1840 edition.
Summary: Maintain in Alma 42:20 the infinitival marker to (“men would not be afraid to sin”), the reading of all the earliest textual sources (including the original manuscript).