Sforno on Numbers 31:27/ספורנו על במדבר ל״א:כ״ז

1וחצית את המלקוח, seeing that this war was a war of revenge for something the enemy had done against the people as a whole, the Torah wanted that the principle of ואכלת את אויביך, “you will consume your enemies,” (Deuteronomy 20,14) be fulfilled. Something similar occurred with David and the loot taken from Amalek (Samuel I 30,26) when he distributed loot he personally had captured from the Amalekites.