In v. 8, he expressed the sum of the law as still applicable. If you love your neighbor as yourself, you are doing right. To love one s neighbor is to keep that ancient law code that God gave us, and whenever we break it, we hate our neighbor in some form. If I dishonor my parents, I hate them. If I take his wife in adultery, that is far from a loving act if his interests matter at all.
Love is the fulfillment of the law, the glove to fit the hand.
Our Lord himself, far from weakening the scope of the law, returned it to the cutting edge. Christ did not annul or lessen the law. He used it to show the corruption of the human heart. The very act of favoritism is breaking the law. It is not treating people similarly.