Albert Barnes Commentary Revelation 16:5

"And I heard the angel of the waters saying, Righteous art thou, who art and who wast, thou Holy One, because thou didst thus judge:" — Revelation 16:5 (ASV)

And I heard the angel of the waters say. The angel who presides over the element of water; in allusion to the common opinion among the Hebrews that angels presided over elements, and that each element was committed to the jurisdiction of a particular angel. (Compare to Barnes on Revelation 7:1).

Thou art righteous, O Lord. In view of the judgments that reddened these streams and fountains with the blood of men, the angel ascribes righteousness to God. These judgments seemed terrible—the numbers slain were so vast—the bloody stream indicated so great slaughter, and such severity of the Divine judgment; yet the angel sees in all this only the act of a righteous God bringing just retribution on the guilty.

Which art, and wast, and shalt be. That is, who is eternal—existing now; who has existed in all past time; and who will exist ever onward. (See Barnes on Revelation 1:8).

The reason why this attribute of God is here referred to seems to be that the mind of the angel adverts to it in the changes and desolations that were occurring around him. In such overturnings among men—such revolutions of kingdoms—such desolations of War—the mind naturally turns to one who is unchanging; to one whose throne is from everlasting to everlasting.

Because thou hast judged thus. You have allowed these wars to occur that have changed rivers and fountains to blood.