Those who deny a natural human right to life subscribe to an amoral system in which all things may be permitted. In such a system, the Nazis would have been acquitted at Nuremberg. The Nazi argued from legal positivism, ie., that which the law permits or prohibits is identical with morality.
Morality deals with justice -- human rights and obligations. The philosophy of legal positivism prevents arguing for human rights outside the legal system per se. Legal systems cannot criticize each other. Legal positivism does not allow argument against Nazi (or Ugandan or Bangladesh or Cambodia) jurisprudence other than a lack of internal consistencies. Under legal positivism, if legal system A, claims that legal system B is immoral it must do so only from a reference to itself. System B does not recognize the validity of system A, so the criticism by system A of system B is correctly disregarded as baseless by system B.
The Nazis leaders used legal positivism to defend themselves at Nuremberg. The only reason, the Nazis claimed, that they found themselves in the defendants' chair at Nuremberg was that they had the misfortune of losing the war.
The Nazis granted that their legal system was different than the Allies, and granted that fundamental German values were different than the Allies, one of which was the supremacy of the Aryan race. They incorporated their values into their laws that included the de-valuing of Jews, Gypsies, and "feeble-minded" relative to Aryans. The Nazis argued, therefore, that the systematic elimination of these groups was, in the German legal system, entirely valid. And, since, under legal positivism, the Allies could not judge the Nazis legal system as invalid, the Allies could not judge the defendants acts as criminal.
Jackson, the lead prosecutor, had to depart from the philosophy of legal positivism and proceed to a higher authority, a new and higher vantage point to prosecute the legal system of another country. He appealed to the basic principles of civilization in order to prosecute the jurisprudence of the Nazi legal system. To transcend human law had to take recourse natural law -- the Creator's law.