King Saul was told to destroy Amalek completely. He did not. Centuries later, according to SAFE0 Rabbah, the Jewish people paid for that moment of misplaced mercy with a genocidal decree.
Rabbi Levi began with a verse from Numbers that reads like a warning written in advance: "If you will not dispossess the inhabitants of the land from before you, those who you leave will be like thorns in your eyes, and like stones in your sides, and they will trouble you in the land you inhabit" (Numbers 33:55). The midrash (rabbinic interpretive commentary) applies this directly to Saul's failure. When Samuel commanded him, "Now go and smite Amalek" (I Samuel 15:3), Saul went to war but could not bring himself to finish the job. He spared Agag, king of the Amalekites: "Saul and the people spared Agag" (I Samuel 15:9).
Samuel's response was devastating. You went out innocent, he told Saul, and you returned guilty. A descendant will survive from this man. That descendant will become a thorn in your eyes and a stone in your sides. And who was that descendant? Haman, the Agagite, who centuries later stood in the court of Ahasuerus and issued the decree "to destroy, to kill, and to eliminate" every Jew in 127 provinces (Esther 3:13).
The midrash draws a straight line from one act of mercy to one act of annihilation. Saul spared one king. That king's bloodline produced the man who tried to end the Jewish people entirely. When everyone saw what Haman had set in motion, they began screaming: "Woe!" And so the Book of Esther opens with that word of anguish hidden in its very first syllable: vayhi, "it was," which the rabbis heard as vai, "woe," for what transpired during the days of Ahasuerus (Esther 1:1).