My opinion.
When people live together in their millions, there has to first an ethic that defines evil, and therefore defines moral, and is applicable to all, including the rulers, and second, an enforcer of that ethic.
If not the first, the whole of the people would rapidly disintegrate, there being no binding of trust that each one's necessities for survival was secure from wanton violation by another. The foundation of any system of moral behavior is the golden rule and all its variations, for if you would not want someone to steal from you, it's evil to steal from them.
The last 6 of the ten commandments govern the first.
If there is no higher law than man then man is a law unto himself. Even though the people agree to live under the common and written law, no man can watch them in their private moments when temptation comes. The only thing that's present is the sense of that which is omniscient, omnipresent and omnipotent.
But more important is that same sense in the rulers, leaders and makers of policy. They have the greatest temptation. If, in their minds, there is no higher power than they, what keeps them from tyranny and evil when all they have to do is not get caught?
The first 4 of the ten commandments govern the second.
Our law is based these assumptions, and the integrity of our country depends on each member holding to them. The constitutions of the states and the federal/national government is founded on that law because the survival of the people depends on our administrators holding to them.
Best I can do.