Yalkut Shimoni on Nach 50:4

"From Ephraim, their root" (Judges 5:14): this is Joshua, as it is said, "And Joshua weakened Amalek" (Exodus 17:13). He rooted his strike into Amalek, as it is said, "For I will utterly blot out the memory of Amalek" (Exodus 17:14). "After you, Benjamin" (Judges 5:14): this is Saul, of whom it says, "and put to death man and woman" (1 Samuel 15:3). And likewise Mordecai. The Holy One, blessed be He, said: it is foreseen and revealed before Me that he is destined to decree to destroy, to slay, and to annihilate all the Jews, young and old (Esther 3:13); therefore "put to death man and woman," to fulfill what is said, "and they shall eat of the fruit of their own way" (Proverbs 1:31).