Was found (heurethē). First aorist passive indicative of heuriskō. See Rev 16:6; Rev 17:6 for the blood already shed by Rome. Rome “butchered to make a Roman holiday” (Dill, Roman Society, p. 242) not merely gladiators, but prophets and saints from Nero’s massacre a.d. 64 to Domitian and beyond.
Of all that have been slain (pantōn tōn esphagmenōn). Perfect passive articular participle genitive plural of sphazō, the verb used of the Lamb slain (Rev 5:9, Rev 5:12; Rev 13:8). Cf. Mat 23:35 about Jerusalem.