Since Romans 4:15 says without Law there is no transgression, how could Cain be charged with the murder of his brother Abel? You cannot violate or transgress a law that does not exist.

Father Adam was created in the image of God. When he sinned, he received a sentence or curse because of his disobedience to the divine will. Thus, a law must have been given him that was sufficiently explicit. Adam and Eve were not prone to sin. They were righteous, surrounded by righteous and perfect conditions, and conscious of their obligations to their Creator. They knew, not vaguely, but precisely, what He had commanded. They were, therefore, without excuse in their transgression.

Surely Adam’s children were well-schooled in righteousness in order to help them be faithful to God and to order their own lives. Yet, sin was now inherent in man. Adam’s children inherited the beginnings of a propensity to disobedience that would never have been there had he not sinned against direct instruction from God.

As for a law, there need be no law in humans to realize that inflicting harm on another being was wrong. Written laws have been created because the farther mankind came from Adam, the more they sinned. They no longer instinctively knew right from wrong,

The very fact that God didn’t exact the punishment of death upon Cain indicated that merciful God understood a degree of innocence was in him, yet he never was permitted to forget his sin.