Is Customary Law on the Prohibition to States to Commit Acts of Genocide Applicable to the Armenian Massacres?

At the time of the massacres commonly known as Metz Yeghérn, did some international unwritten rule exist regarding what is later defined as genocide? If not, does the subsequent coming into being of a customary, peremptory rule have any effect today for Turkey and for all the other contemporary States? The absence of a customary rule prohibiting States from committing genocides at the time of Metz Yeghérn is demonstrated through the analysis of rulings of the International Court of Justice, the preparatory works of the International Law Commission's Articles on the Law of the Treaties and on State Responsibility, the preparatory works of the 1948 Genocide Convention and the behavior of the different States involved during and after the 1915-1916 massacres. The current peremptory rule on the prohibition of genocide is neither retroactive, nor applicable to many of the current behaviors of modern Turkey vis-à-vis Armenians and/or those past events, whose illegality is often questioned on different grounds.