Adam Clarke on James 2:8

The royal law - Νομον βασιλικον. This epithet, of all the New Testament writers, is peculiar to James; but it is frequent among the Greek writers in the sense in which it appears St. James uses it. Βασιλικος, royal, is used to signify any thing that is of general concern, is suitable to all, and necessary for all, as brotherly love is. This commandment; Thou shalt love thy neighbor as thyself, is a royal law, not only because it is ordained of God, and proceeds from his kingly authority over men, but because it is so useful, suitable, and necessary to the present state of man; and as it was given us particularly by Christ himself, Joh 13:34; Joh 15:12, who is our King, as well as Prophet and Priest, it should ever put us in mind of his authority over us, and our subjection to him. As the regal state is the most excellent for secular dignity and civil utility that exists among men, hence we give the epithet royal to whatever is excellent, noble, grand, or useful.