The Royal Law, the Whole Law, and Mercy over Judgment
James names Leviticus 19:18—“Thou shalt love thy neighbour as thyself”—the royal law. Partiality violates that law. He then cites the commands against adultery and murder to explain that the law’s authority is unified in the one God who speaks it. The section concludes with judgment and mercy: those who show no mercy face merciless judgment, while mercy triumphs over judgment.
Where this fits in the story: Leviticus 19 places love of neighbor inside a chapter filled with impartial justice, honest dealings, care for the poor, and holiness. James is not plucking a slogan from its context; his concern with favoritism, speech, wages, and neighbor-care resonates strongly with the same ethical world.