6 Then the Lord said to Cain, “Why are you angry, and why is your expression downcast? 7 Is it not true that if you do what is right, you will be fine? But if you do not do what is right, sin is crouching at the door. It desires to dominate you, but you must subdue it.” 8 Cain spoke to his brother Abel.21 While they were in the field, Cain attacked his brother Abel and killed him. 9 Then the Lord said to Cain, “Where is your brother Abel?” And he replied, “I don’t know! Am I my brother’s guardian?” 10 But the Lord said, “What have you done? The voice of your brother’s blood is crying out to me from the ground!
1 John 3:12
10 By this the children of God and the children of the devil are revealed: Everyone who does not practice righteousness—the one who does not love his fellow Christian—is not of God. 11 For this is the gospel message that you have heard from the beginning: that we should love one another, 12 not like Cain who was of the evil one and brutally murdered his brother. And why did he murder him? Because his deeds were evil, but his brother’s were righteous. 13 Therefore do not be surprised, brothers and sisters, if the world hates you. 14 We know that we have crossed over from death to life because we love our fellow Christians. The one who does not love remains in death.
Notes and References
"... Cain is portrayed in 1 John as the original man-hater and man-murderer, and this interpretation of Cain’s story goes beyond what is reported in Genesis 4:4-9: Cain’s hatred in 1 John is the “original hatred” that comes from darkness. A reasonable conclusion, therefore, is that the author’s use in 1 John 3:12 derives from the tradition available to him, in which Cain’s action in killing his brother is intensified from an act of simple killing borne of brotherly jealousy, as portrayed in the Torah, to a manifestation of primaeval evil, done by a person who himself stands as the all-time personification of evil itself, such as we find in Philo and Josephus. Further, we have noticed that even in the LXX, Genesis 4:8 uses the word which simply means “kill”, to translate the Hebrew ַ וַ יּה גְר הוּ, “and he killed him”, which has a similar meaning, even if the Greek is slightly intensified compared to the Hebrew ... To say, as 1 John 3:12 does, that Cain ἔσφαξεν τὸν ἀδελφὸν αὐτοῦ, “slaughtered his brother, is a great step-up from the simple LXX statement ..."
Armitage, Chris Atonement and Ethics in 1 John: A Peacemaking Hermeneutic (p. 220) St Mark's National Theological Centre, 2014