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Shaul's mistake led to Haman's birth which endangered the Jewish People. Through undoing of his mistake the Jewish People were saved from Haman. Shaul followed his intellect in saving the animals of Amalek for sacrifice, but Shmuel reprimanded him that listening to Hashem - Kabolas Ol is greater than serving Hashem with our mind - though that is necessary as well. The Jewish People in the time of Haman were willing to give up their lives for their belief in Hashem, that is why they were called Yehudim. That self sacrifice beyond reason led to the downfall of Haman. We celebrate it now as well with Ad Dlo Yada - not just serving Hashem with our intellect, but "until one does not know" beyond all of our soul powers, even intellect.