Bereshit 

בראשית

The Sages elaborate on the first murder. Why did Cain kill Abel? What was Cain’s reaction after the murder, and what was his punishment?  

“Cain said to Abel his brother” (Genesis 4:8). What did Cain say to him?<footnote data-num="18"/> Cain said to Abel: Shall we divide the world between us? Abel said to him: Yes. Abel took his flocks to graze and Cain took the land to cultivate, and they agreed that neither of them would have any claim against the other. When Abel took his flocks, he began to shepherd the flocks, but Cain would pursue him from mountain to valley and from valley to mountain.<footnote data-num="19"/> This went on until they encountered each other and fought. Abel triumphed over Cain and Cain fell beneath him. When Cain saw that Abel had prevailed he began to scream: Abel my brother, do not do me harm. Abel pitied him and released him, and Cain rose and killed him. Once Cain killed him, he said: I will flee from my father and my mother, who will ask me about him, as there is no one in the world other than me and him. Immediately, the Holy One, blessed be He, appeared to Cain and said to him: You can flee from your parents, but you cannot flee from Me… “Where is Abel your brother?” (4:9). God said to him: Woe unto Abel, as he pitied you and didn’t kill you when you fell beneath him, and you rose and killed him. How did Cain kill him? He made several wounds and several injuries with a stone, in Abel’s arms and legs, as Cain did not know from where his life departs. Cain continued striking Abel until he reached his neck…Cain said to God: Master of the Universe, I did not know and I had never seen anyone killed in my life. How could I know that if I smite him with a stone he would die? Immediately, He responded to Cain: “And now, cursed are you from the ground that opened its mouth to take your brother’s blood from your hand. When you cultivate the ground, it shall not continue giving its strength to you” (4:11–12). Cain said before Him: Master of the Universe, do You have informants who inform on people before You? My father and my mother are on earth and do not know that I killed Abel, and You are in the heavens; from where do You know? God said to him: Fool! I bear the entire world…Cain said to Him: You bear the entire world and You are unable to bear my iniquity? Is my sin too great to bear? (4:13). God said to him: Since you have repented, depart and be exiled from this place…When Cain departed, every place that he would go, the land would quake beneath him and the beasts and the animals would quake and say: What is this? They would say to each other: Cain killed his brother Abel, and the Holy One, blessed be He, decreed in his regard: “Restless and itinerant you shall be” (4:12). <source>(<i>Tanĥuma</i>, <i>Bereshit</i>) </source>