Cain killed Abel to end the life/knowledge of God in the earth then he went ahead to build another civilization entirely, untill God gave Adam another Son and that Son had another son before the name of the lord came back again into the earth Genesis 4:5 KJV But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell. Genesis 4:8 KJV and Cain talked with Abel his brother: and it came to pass, when they were in the field, that Cain rose up against Abel his brother, and slew him. Genesis 4:26 KJV And to Seth, to him also there was born a son; and he called his name Enos: then began men to call upon the name of the LORD. 1 John 3:12 KJV Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother's righteous.