William Burkitt on Revelation 16:5

These verses seem to be a key to let us into the right understanding of the verses immediately foregoing; and clearly show, that as the idolatrous enemies of the church did shed the blood of saints and martyrs, so should theirs be shed: which is called their having blood to drink.

Here note, 1. The approbation which the angel gives of the equity and righteousness of God's judgments upon these bloody persecutors. Righteous art thou, O Lord, for thou hast given them blood to drink. Where the retribution which God makes of blood for blood, shows the righteousness of God in his judgments inflicted; and also declares that Almighty God doth sometimes so suit his judgments to the sin committed, that persons may read their sin in their punishment. As these persecutors thirsted after blood, so here they get blood to drink.

Note, 2. How the angel in the fifth verse, and the other in the seventh verse, do both rejoice in the execution of God's judgments, and in the terrible revenge of the blood of saints. For thus they sing, Righteous art thou, O Lord, and just are thy judgments.