What was Cain's respone to God's question when he killed his brother?

There are several ways of defining the conflict between Cain and Abel. In Genesis, Cain kills Abel because God was pleased with his sacrificial offering, but rejected Cain's offering.

Leon R. Kass (The Beginning of Wisdom: Reading Genesis) points out that the birth of Abel is not celebrated by his mother. Abel, introduced only as "his brother, Abel" seems to be an afterthought. There is no described relation to his mother, only to Cain. If he is important only or mainly as Cain's brother, arguably he only exists in the Book of Genesis to play his role in the drama that follows.

Asaph Sagiv believes that there is an Egyptian antecedent to the story of Cain and Abel. He suggests that the Cain and Abel story represents a biblical countermyth to the Egyptian story of Osiris and his brother Seth. In that story, Seth, an evil god of the nomads, kills Osiris, the Egyptian god of the earth's fertility. In Genesis, Cain (an Osiris figure) offers a cult sacrifice (Abel) to the earth; the Lord curses the ground for accepting the victim's blood and banishes Cain from his divine presence; and Seth appears as a substitute for the nomadic victim whose sacrifice the biblical God preferred.

There is also a possible nationalistic theme to the story. The Hebrews identified themselves as keepers of sheep, while their neighbours to the west and north were farmers, or tillers of the ground. We find that the innocent victim was a keeper of sheep, while the evil Cain was a farmer.