The Hebrew of (Genesis 4:8) is notoriously fragmentary: "Cain said to Abel his brother. And it came to pass, when they were in the field, Cain rose up against Abel his brother and killed him." What did Cain say? The Torah leaves a conspicuous silence. Targum Pseudo-Jonathan fills it with a full theological debate.
Cain's position: no justice, no judge
Cain speaks first. He says, "I perceive that the world was created in goodness, but it is not governed according to the fruit of good works, for there is respect to persons in judgment; therefore it is that thy offering was accepted, and mine not accepted with good will."
Cain's grief has hardened into philosophy. The universe looks rigged to him. God has favorites. Righteousness does not pay. His offering was rejected not because of its quality, but because God picks winners.
Abel's position: there is a Judge
Abel answers calmly: "In goodness was the world created, and according to the fruit of good works is it governed; and there is no respect of persons in judgment; but because the fruits of my works were better than thine, my oblation, before thine, hath been accepted with good will."
Abel concedes that his offering was better. He does not gloat. But he insists that God judged fairly, not capriciously. Acceptance tracks effort.
Cain's escalation
Cain will not be consoled. He pushes further: "There is neither judgment nor Judge, nor another world; nor will good reward be given to the righteous, nor vengeance be taken of the wicked."
This is Cain's real sin — not just jealousy, but the denial of the moral order of the universe itself. He becomes, in the Targumist's reading, history's first heretic. Abel answers with the affirmation: "There is a judgment, and there is a Judge; and there is another world, and a good reward given to the righteous, and vengeance taken of the wicked."
And then: "Because of these words they had contention upon the face of the field; and Kain arose against Habel his brother, and drave a stone into his forehead, and killed him."
Cain kills his brother over a theological argument. A stone to the forehead, to silence the voice that insisted the world made moral sense. The Targumist is showing us that murder is often, underneath, an argument about whether God is paying attention — and an attempt to prove He is not.