Revelation 16:6

they have shed the blood of saints and prophets
Those who had shed the blood of saints and prophets are the “peoples, multitudes, nations, and tongues” upon which the Harlot sits (Rev. Rev. 17:15+) for she is “drunk with the blood of the saints and with the blood of the martyrs of Jesus” (Rev. Rev. 17:6+). The godless have shed the blood of saints and prophets throughout history (1K. 1K. 18:4; 1K. 19:4; 2K. 2K. 24:4; 2Chr. 2Chr. 24:21; Ps. Ps. 79:1-4; Jer. Jer. 2:30; Jer. Jer. 26:23; Lam. Lam. 4:13). At the First Coming of Christ, Israel was especially guilty of shedding the blood of her own righteous (Mtt. Mat. 21:35-36; Mat. 23:35; Luke Luke 11:49-53). Since the rejection of Messiah Jesus and the going forth of the gospel to the whole world, over 1900 years have transpired and the toll of the shed blood of the saints has escalated dramatically. It has been said in our own century that more have died for the cause of Christ than any previous century. Yet the persecutions of history will pale in comparison with that which befalls the saints in the Tribulation when the Beast (Dan. Dan. 7:21, Dan. 7:25; Rev. Rev. 13:7+, Rev. 13:10+), his image (Rev. Rev. 13:15+), and the Harlot (Rev. Rev. 17:6+; Rev. 19:2+) all slaughter the saints during Satan’s final attempt to overthrow the plan of God (Rev. Rev. 12:17+). This is why the book of Revelation has always been precious to the persecuted Church, for its pages are sprinkled with the blood of martyrs (Rev. Rev. 6:9-10+; Rev. 7:14+; Rev. 11:7+; Rev. 12:11+, Rev. 12:17+; Rev. 15:2+; Rev. 20:4+). See #20 - Saints.

You have given them blood to drink
Because they have shed the blood of the saints and prophets, in a wry twist of God’s hand of judgment, He gives them literal blood to drink. This is similar to how He stones those who are guilty of blasphemy. See commentary on Revelation 16:21.

I will feed those who oppress you with their own flesh, and they shall be drunk with their own blood as with sweet wine. All flesh shall know that I, the LORD, am your Savior, and your Redeemer, the Mighty One of Jacob. (Isa. Isa. 49:26) [emphasis added]

For it is their just due
Ἄξιοι γάρ εἰσι [Axioi gar eisi] , For worthy they are. By their works, they have demonstrated their worthiness of this righteous punishment.