The law given to Adam was to Him alone. No one aftere the fall can duplicte that sin so that law is not more. The only law given after that to all mankind was the law of Moses.
I think you forget that everyman even those who have never read of the Law of Moses has the Law writen on their hearts. I don't believe there has ever been a time in history when men didn't know right from wrong. Certainly conscience is not possible with out knowing what is right and what is wrong. Cain other wise should not have been held responsible for the death of Able but he was.
MB