27 “Nevertheless, God shall break in pieces the heads of His enemies, the scalp of hair of men walking on in their transgressions”: that is, too much exalting themselves, being too proud in their transgressions: wherein at least they ought to be humble, saying, “O Lord, be merciful to me a sinner.” But He shall break in pieces their heads: for he that exalts himself shall be humbled. And thus though even of the Lord be the outgoing of death: nevertheless the same Lord, because He was God, and died after the flesh of His own will, not of necessity, “shall break in pieces the heads of His enemies:” not only of those who mocked and crucified Him, and wagged their heads, and said, “If Son of God He is, let Him come down from the Cross;” but also of all men lifting up themselves against His doctrine, and deriding His death as though it were of a man.
For that very same One of whom has been said, “Others He saved, Himself He cannot save,” is the “God of our healths,” and is the “God of saving men:” but for an example of humility and of patience, and to efface the handwriting of our sins, He even willed that the outgoing of death should be His own, that we might not fear that death, but rather this from which He has delivered us through that. Nevertheless, though mocked and dead, “He shall break in pieces the heads of His enemies,” of whom He says, “Raise me up, and I shall render to them:” whether it be good things for evil things, while to Himself He subdues the heads of them believing, or whether just things for unjust things, while He punishes the heads of them proud.
For in either way are shattered and broken the heads of enemies, when from pride they are thrown down, whether by humility being amended, or whether unto the lowest depths of hell being hurled.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)