What does Revelation 16:6 mean?

The angel who proclaims that God is holy and just says the wicked received what they deserved. They had shed the blood of saints and prophets, and therefore God gave them blood to drink. The wicked had received ample warnings about the coming wages of their sins.

When Moses handed down the civil law to Israel, he decreed the principle of reciprocity, that a murderer must pay for his crime with his own life. Numbers 35:33 states: "You shall not pollute the land in which you live, for blood pollutes the land, and no atonement can be made for the land for the blood that is shed in it, except by the blood of the one who shed it." Earlier Moses had warned the Israelites that they could not disobey the Lord without facing the consequences. "Be sure your sin will find you out," he said (Numbers 32:23). Those who disobey the Lord by practicing idolatry in the tribulation will discover the truth of Moses' warnings.