Gary North's critique of natural law is to the point, (Moses and Pharaoh, pg. 236-246):

"Such a conception of natural law rests on the assumption of a universal human logic, the universal applicability in history of that logic, and the universal recognition of this logic and its universal applicability by all reasonable men. It assumes, in short, the neutrality of human thought.

"The Bible explicitly denies any such neutrality. Men are divided into saved and lost, keepers of God's covenant and breakers of God's covenant. There is no agreement between the two positions. Human reason, unaided by God's revelation - untwisted by God's grace cannot be expected to devise a universal law code based on any presumed universal human logic. What mankinds' universal reason can be expected to do is to rebel against God and his law. Ethical rebels are not logically faithful to God.