God's Judgment on Unjust Rulers

"Woe to those who decree iniquitous decrees, 

And the writers who keep writing oppression."

-Isaiah 10:1 

As a lover of God's Law, I often hear remarks saying that, since we are "under grace, not under law" (a misreading of Romans 6:14), the civil magistrate is under no obligation to the word of God in the execution of his office. In fact, any source of law except for the Bible is preferred, including the humanistic law under which we Americans are forced to live. 

Sometimes the problem is a belief that the Law ended with the crucifixion of Jesus. That is just bad hermeneutics. Other times, the person - and remember that I am speaking of professing Christians - will point to something in that Law and claim that it would be horrible to live by such a standard today. And they say that, completely oblivious to the implied blasphemy, as if God were unjust in His moral standards!  

Yet we have the verse above, Isaiah 10:1, in which God calls the actions of some magistrates "iniquitous." By what standard is an action iniquitous? Is the law of the state to be judged by its own standards? Surely such circularity would always confirm the morality of that law. Of course not! It must be judged by the objective standards of God. And where do we find those standards? In His law, recorded for us in the Bible.