Revelation 16:5-7 "And I heard the angel of the waters say, Thou art righteous, O Lord…because Thou hast judged thus. For they have shed the blood of saints and prophets, and Thou hast given them blood to drink; for they are worthy. And I heard another out of the altar say, Even so, Lord God Almighty, true and righteous are Thy judgments"
"This is the only place in the Revelation where the altar is said to speak, though we have heard before of a voice from the horns of the altar (Rev. 9:13). In Revelation 8:5 we find the altar connected with the prayers of the saints, and as a result of their prayers fire is taken from off the altar and cast into the earth. The fire is a symbol of purging and purification, and the voice from the horns of the altar comes also as a result of the prayers of God’s elect and releases a mighty deliverance in the lives of God’s called and chosen remnant. Here in our text the altar is emphasizing that God’s judgments in the midst of His people are true and righteous, that is, they are just and purposeful. The message is just this — God is doing what He has to do in order to correct what is wrong in His people, and what He does is just and redemptive, bringing deliverance and restoration."
"Another strange statement is that because they have shed the blood of saints and prophets, God is giving them blood to drink: for they are worthy. That last phrase simply means "for they deserve it!" We don’t like to think of God giving people what they deserve, but in this case that is exactly what He is doing. Shed blood signifies death. That is what they deserve to drink, death to themselves. The reference is to the fact that the church systems have slain, both figuratively and literally, the righteous men and prophets sent to speak the words of God to them through the ages. Thousands have been the martyrs slain by the church systems in ages past and even in recent times! They chose to drink from the human sources of this old world. Thus they wickedly rejected the living streams of water flowing out from God’s chosen vessels! So now He is causing them to drink a full dose of the death they have chosen! Let them drink blood! Oh, yes! They deserve it! It is not vindictiveness — it is correction! The Spirit is revealing to them the awful depths of the death that flows in the streams of man! And it is for their good, for their deliverance, and ultimate perfection!"
Much more in the link:
"It should be obvious to every believing heart that the darkness that fills the beast’s kingdom is not a literal darkness like that in Egypt in the day of Moses, but truly spiritual darkness. Hear it! “Understanding is a WELLSPRING OF LIFE unto him that hath it.” Certainly one reason so many millions of Christians are living in carnality and immaturity, far beneath their privileges in Christ today, is because they neither know who, where, or why they are. Truly a great and dreadful darkness is upon the kingdom of Babylon! This condition is explained by Paul in his epistle to the Ephesians. “This I say therefore, and testify in the Lord, that ye henceforth walk not as other Gentiles walk, in the vanity of their mind, having the understanding darkened, being alienated from the life of God through the ignorance that is in them, because of the blindness of their heart” (Eph. 4:17-18). In this passage we learn, among other things, this one simple truth: Darkness IS IGNORANCE. In its spiritual meaning, to be in darkness is to be in ignorance! Ignorance is a condition of the mind, in which the mind lacks understanding. The inspired apostle tells us that men have their “understanding darkened” because of the “ignorance” that is in them! It means that either they don’t know much, or the information they have been given is incorrect! And it is this condition of spiritual ignorance that causes the whole world of men, and vast numbers of the Lord’s precious people, to walk in darkness, alienated from the abundant life that is in Christ! Oh! How many of the Lord’s people walk in spiritual darkness, or spiritual ignorance, because the teaching they have been fed by those who are supposedly the messengers of God is not the truth at all! Men are in darkness because the blindness of their hearts has kept them from the glorious Light of Christ penetrating into their lives!"