And I heard another out of the altar say, Even so, Lord God Almighty, true and righteous are thy judgments.
You sometimes hear it said, “Justice requires God to do this.” I’ve probably used this expression myself, though it is semantically improper. The human language staggers when we try to use it to describe God. The prophets of the Old Testament and the apostles of the New put such pressure on language that words groan and squeak under the effort to tell the story. We must remember that justice is not something outside of God to which God must conform. Nothing ever requires God to do anything. If you have a god who is required to do anything, then you have a weak god who has to bow his neck to some yoke and yield himself to pressure from the outside. Then justice is bigger than God. But that is to think wrongly.
All God’s reasons for doing anything lie inside of God. They do not lie outside of God to be brought to bear upon Him. They lie inside of God—that is, they are what God is. And God’s reasons for doing what He does spring out of what God is….
God is justice, and God will always act justly—not by compulsion from the outside but because that’s the way He is Himself. Justice must always prevail because God is the sovereign God who will always prevail. AOG061-063
Lord, may we remember that You are not compelled to do anything, but that You will always be fair because You are justice itself. Amen. 
Then the apostle John heard the altar saying, “Yes, O Lord God, the Almighty, true and righteous are Your judgments.” The personified altar echoes the sentiments of the angel with words similar to 15:3. It may be that the very altar under which the saints were earlier seen praying for vengeance (6:9–11) now affirms that God’s true and righteous judgments are the answer to those prayers.
That God’s judgments are true and righteous is the constant teaching of Scripture. They are not like the capricious judgments associated with false pagan gods. In Genesis 18:25 Abraham asked rhetorically, “Shall not the Judge of all the earth deal justly?” David wrote in Psalm 19:9, “The judgments of the Lord are true; they are righteous altogether,” while in Psalm 119:75 the psalmist added, “I know, O Lord, that Your judgments are righteous.” Paul wrote of “the day of wrath and revelation of the righteous judgment of God” (Rom. 2:5). In 19:1-2 John “heard something like a loud voice of a great multitude in heaven, saying, ‘Hallelujah! Salvation and glory and power belong to our God; because His judgments are true and righteous; for He has judged the great harlot who was corrupting the earth with her immorality, and He has avenged the blood of His bond-servants on her.’ ”
16:7 The altar probably symbolizes the souls of martyred saints (6:9). They had waited long and patiently for their persecutors to be punished.
16:7 altar. The personified altar echoes the words of the angel, reinforcing the truth that God is just in all judgment (19:1, 2; cf. Ge 18:25; Ps 51:4; Ro 3:4).
16:7 I heard the altar saying Refers to those that dwell beneath the altar—the martyrs. This is the ultimate vindication in response to their query in 6:10.
righteous God has avenged the blood of the martyrs. The murderers received righteous retribution.
 Tozer, A. W., & Eggert, R. (2015). Tozer on the almighty god: a 365-day devotional. Chicago, IL: Moody Publishers.
 MacArthur, J. F., Jr. (2000). Revelation 12–22 (p. 143). Chicago: Moody Press.
 MacDonald, W. (1995). Believer’s Bible Commentary: Old and New Testaments. (A. Farstad, Ed.) (p. 2373). Nashville: Thomas Nelson.
 MacArthur, J. F., Jr. (2006). The MacArthur study Bible: New American Standard Bible. (Re 16:7). Nashville, TN: Thomas Nelson Publishers.
 Barry, J. D., Mangum, D., Brown, D. R., Heiser, M. S., Custis, M., Ritzema, E., … Bomar, D. (2012, 2016). Faithlife Study Bible (Re 16:7). Bellingham, WA: Lexham Press.