What does Revelation 16:5 mean?
The apostle John reports in this verse that he heard an angel in charge of the waters—the waters that have just become blood—extol God. The angel addresses God as, "O Holy One, " and declares that He is just and eternal.As the Holy One, God cannot think or do anything wrong or sinful. So, what He does in judging the wicked is right and in keeping with His abhorrence of evil. As the eternal God, He has always been holy and just, is holy and just now, and always will be holy and just. He never accuses the innocent and never excuses the guilty. The angel understands clearly that God is the source of the judgments. He agrees fully with God's action of turning the waters into blood. He yields his administration of the waters into God's righteous hands.
In the fifth seal judgment, the martyrs had begged God to punish their murderers (Revelation 6:9–11). Now the time of vengeance on their behalf has arrived.