Albert Barnes Commentary Revelation 16:6

"for they poured out the blood of the saints and the prophets, and blood hast thou given them to drink: they are worthy." — Revelation 16:6 (ASV)

For they have shed the blood of saints. The nations here referred to. They have been engaged in scenes of bloody persecution, and this is a just recompense.

And prophets. Teachers of religion; ministers of truth. It is not necessary to understand the word prophets here in its technical sense as meaning those who are raised up by God and sent forth as inspired men, but it may be understood in its more common meaning in the New Testament as meaning teachers of religion in general. (See Barnes on Romans 12:6; 1 Corinthians 14:1).

And thou hast given them blood to drink. Namely, by turning the streams and fountains into blood, Revelation 16:4. Blood had been poured out in such abundance that it seemed to mingle with the very water that they drank. This was a recompense for their having, in those very regions, poured out so much blood in persecuting the saints and prophets— the pious private members of the church, and the public teachers of religion.

For they are worthy. That is, they deserve this; or, this is a just recompense for their sins. It is not intended that those who would thus suffer had been individually guilty of this, or that this was properly a punishment on them; but it is meant that in those countries there had been bloody persecutions, and that this was a fit recompense for what had there occurred.