Deuteronomy 32:42 - Matthew Poole's English Annotations on the Holy Bible

Of the captives; whom my sword hath sorely wounded, though not utterly killed. From the beginning of revenges upon the enemy, i.e. when once I begin to revenge myself and my people upon mine and their enemies, I will go on and make a full end. Or, with the head, or with the blood of the head, i.e. of the chief or chiefs, of the revenges of the enemy, i. e. of the revengeful or malicious enemy of God and of his people. The noun substantive is oft put for the adjective; as Genesis 17:5, a multitude of nations is put for many nations, Romans 4:17 Genesis 45:22, changes of raiment, i.e. changeable raiment; and Psalms 99:4, the king's strength, i.e. the strong and mighty king; and so here, the revenges of the enemy, i.e. the revengeful enemy. And by the head may be here understood either the devil, or the beads and rulers of those empires which were enemies to God's people. Or, of the head shall be the revenges upon the enemies, i.e. I will take vengeance upon all mine enemies, yea, upon the head or heads of them.