I believe you were looking in the wrong place. The passage is:
"When He opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the testimony which they held. And they cried with a loud voice, saying, "How long, O Lord, holy and true, until You judge and avenge our blood on those who dwell on the earth?" Then a white robe was given to each of them; and it was said to them that they should rest a little while longer, until both the number of their fellow servants and their brethren, who would be killed as they were, was completed" (Revelation 6:9-11).
The souls are martyred Christians and they are asking God how long it would be until their deaths would be avenged by God (Romans 12:19). The answer was that they would need to wait a bit longer because there were other Christians who would also be martyred. God was waiting until this happens before taking vengeance for all the deaths at once.