Who were slain is ἐσφαγμένων [esphagmenōn] , perfect passive participle, ones having been violently murdered.1 The city is guilty of great religious persecution. She is identical to the Harlot, who drinks the blood of the saints (Rev. Rev. 17:6+). See One or Two Babylons? Since she is the mother of all harlots (Rev. Rev. 17:5+), the blood of the godly shed by all her daughter harlots is put to her account. One such daughter harlot was Jerusalem in the days of Jesus (Luke Luke 11:47-51; Acts Acts 7:52; 1Th. 1Th. 2:15). Many other daughter harlots have been birthed from her. At the bottom of her cup (Rev. Rev. 17:6+) is the blood of Abel (Gen. Gen. 4:10). Her cup continues to fill this very hour. See commentary on Revelation 17:6.
So far as this present world is concerned, the general verdict of mankind, sustained by the great current of human history for 6,000 years is against the faith and testimonies of the saints, apostles, and prophets of God. To the general population of the earth their profession stands branded as mere hallucination and lies. But at last their vindication comes. When the vaunted wisdom, and progress, and experiments of unregenerate man are consummated, and there is nothing to show from it but a valley of burning cinders and desolation, with the whole earth from highest kings to meanest subjects howling in helpless lamentations, terror, and despair, history will have added its seal to all that the saints, apostles, and prophets have said and maintained. Then will their judgment have been judged out of that world which despised and persecuted them, and spurned their hated pessimism for more flattering philosophies.2
1 Frederick William Danker and Walter Bauer, A Greek-English Lexicon of the New Testament and Other Early Christian Literature (Chicago, IL: Uersity of Chicago Press, 2000), 796.
2 J. A. Seiss, The Apocalypse: Lectures on the Book of Revelation (Grand Rapids, MI: Zondervan Publishing House, 1966), 418.