This pronoun refers to those mentioned in Revelation 16:2: “the people who bore the mark of the beast and worshiped its image.” The angels poured the bowls of God’s wrath upon sea and rivers not because the waters had done something to attract his displeasure, but instead because these were part of the creation God had entrusted to mankind—and man had rebelled and continued to rebel in the face of the proclamation of the gospel (Revelation 14:6–7), and persisted in worshipping the beast and its image as well as wearing its mark (Revelation 14:9).
6 For they have shed the blood of saints and prophets, and you have given them blood to drink. It is what they deserve!”