Genesis never says why.
Two brothers bring gifts to God. Cain brings produce from the ground. Abel brings the firstborn of his flock, and the fat portions. God accepts Abel and his gift. He does not accept Cain and his. Genesis never gives the reason, not to Cain and not to us.
That gap is hard to leave alone. Abel's gift gets more detail, and many readers take that as the hint. Rashi, drawing on earlier rabbis, said Cain brought ordinary produce and Abel the choicest animals. The letter to the Hebrews says Abel offered by faith. Both answers come from outside Genesis 4. Inside it, Cain stands with a verdict and no explanation.
Anger came right after the verdict.
The next thing we learn is that Cain was very angry, and his face fell. Most readers call this envy, and the order of events makes that fair: favor to Abel, then anger in Cain, then Abel dead. But envy is the reader's word. The story names anger and a face.
The psychologist Leon Festinger proposed that people judge themselves by comparing with others, especially when they are unsure where they stand, and that coming out worse can bring painful feelings. Cain's situation resembles that setup: no reason given, and a brother beside him whose gift was accepted.
God asked before the field.
Between the anger and the field, there is a conversation. God asks Cain why he is angry and why his face has fallen. Then: if you do well, will you not be accepted? If you do not, sin is lying at the door.
The last line is harder. Something desires Cain, and he will rule over it. The pronouns could point to sin at the door, or to Abel. Either way, the anger is met with a question and treated as something Cain can still answer. The story treats the feeling and the act as two different things, with room between them.
No reply from Cain comes next. The following line says only that he spoke with Abel. What they said is not recorded. Then, in the field, Cain rose up against his brother and killed him.
He turned on his brother, not the judge.
The verdict came from God. The one Cain rose against was Abel. Abel never speaks in this story, and we are shown nothing he did to Cain. Something could have passed between them in that lost conversation. But the sequence suggests the anger landed on the brother who was favored.
After the killing, Abel's only voice is his blood, crying from the ground. God asks where Abel is. Cain says he doesn't know, which is a lie, and asks whether he is his brother's keeper. The question does not end the matter. The blood is still crying.
