Abimelech ruled over Israel for three years (Judges 9:22). Aggadat Bereshit uses this strange opening — about a king in the book of Judges — to arrive at the first murder. The path runs through broken loyalty. Proverbs warns: "Your friend and your father's friend do not forsake" (Proverbs 27:10). David took this seriously when he showed kindness to Hanun, son of his father's ally. Hanun repaid him by humiliating David's ambassadors — shaving their beards, cutting their garments. The breach of covenant became the beginning of a war.
This is the midrash's way into Cain and Abel. The first murder was also a breach of the most intimate covenant possible — between brothers. Cain had everything: the firstborn status, the work of the ground, the legacy of Adam. And when God accepted Abel's offering and not his, something curdled in him. The rabbis asked what the two brothers argued about before the murder. Their answers varied: land, women, the location of the Temple. All variations on the same theme — what we believe should be ours, and what we're willing to do when someone else has it.
God warned Cain directly: "If you do not do well, sin is crouching at the door. Its desire is for you, but you must rule over it" (Genesis 4:7). The door image is vivid — sin waiting just outside the threshold, patient, ready to enter. Cain did not master it. He invited it in. And the first blood spilled on this earth was not from an animal sacrifice but from a brother who had been warned, explicitly, that he was about to make the worst decision in human history.