21 For what follows? “They will be captious against the soul of the righteous”. Why will they be captious? Because they can find no true ground of accusation. For how were they captious against our Lord? They made up false accusations, because they could not find true ones. “And will condemn the innocent blood.” Why all this takes place, he will show in the sequel.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)