There are no differences in verses 40 through 44 from the same verses in Matthew 5, save for the shift from “ye have heard” to “it is written.” 3 Nephi 12:45 removes the final phrase from Matthew 5:45: “and sendeth rain on the just and on the unjust.”
These instructions were certainly tailored to the conditions of the Roman rule. The intent of the verses is to teach Jesus’s listeners how to treat their enemies. In both the Old and New Worlds, Christians had lived in a society where they were surrounded by enemies, although that condition was much more recent for the Nephites.
Thus, the conclusion is in verses 43 and 44. The relationship of the community of their enemies is to love them rather than hate them. This principle is perhaps exemplified in Alma 31:5: “And now, as the preaching of the word had a great tendency to lead the people to do that which was just—yea, it had had more powerful effect upon the minds of the people than the sword, or anything else, which had happened unto them—therefore Alma thought it was expedient that they should try the virtue of the word of God.”