Where before there was one under the control of evil, now there are two. Evil propagates by contagion. But the followers of Jesus must go further. Duty is not enough. Duty obeys the rules, but love grasps opportunities.
Duty acts under constraint, love is spontaneous and therefore gracious.
Duty expects to be recompensed or at least recognized, love expects nothing in return. In the Old Testament, however, to be perfect means to be completely loyal and is a normal human virtue, but mercy is the very character of God. The son must inherit the attributes of his Father. Generosity in giving must be matched by generosity of judgement. He who gives and forgives sparingly receives sparingly.