Rutherford gives three arguments that the power of the magistrate is not absolute or unlimited.
“Arg. 1.—He who, in his first institution, is appointed of God by office, even when he sitteth on the throne, to take heed to read on a written copy of God’s law, that he may ‘learn to fear the Lord his God, and keep all the words of this law’ … Arg. 2.—The power that the king hath (I speak not of his gifts) he hath it from the people who maketh him king, as I proved before: but the people have neither formally nor virtually any power absolute to give the king … Arg. 3.—All royal power, whereby a king is a king and differenced from a private man, armed with no power of the sword, is from God. But absolute power to tyrannise over the people and to destroy them is not a power from God; therefore there is not any such royal power absolute.”
Why, then, Rutherford, do people say that the king is a living law?
First, “because men are averse to good laws, therefore there was need of a ruler,” namely to compel. Second, the law per se is “the reason or mind, free from all perturbations of anger, lust, hatred, and cannot be tempted to ill,” so that the king “cometh by office out of himself to reason and law.” Third, “Justice is more perfect than a just man … so the nearer the king comes to a law, for the which he is a king, the nearer to a king.”
Lex, Rex, Q22 [101-102]