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.
It is probably fair I give you my understanding of Revelations 6:9-11, as you quoted it to me.
These are the saints and prophets murdered by the Pharisees.
Jesus had promised that He would avenge them:
Luke 18:7 And shall not God avenge his own elect, which cry day and night unto him, though he bear long with them?
8 I tell you that he will avenge them speedily
Among them are those that were slain previously, but it is that generation of pharisees and their followers that will pay:
Mathew 23:35 That upon you may come all the righteous blood shed upon the earth, from the blood of righteous Abel unto the blood of Zacharias son of Barachias, whom ye slew between the temple and the altar.
36 Verily I say unto you, All these things shall come upon this generation.
Matthew 27:25
Then answered all the people, and said, His blood be on us, and on our children.
Notice that the Angel said "they should rest a little while".
That means a little while, not thousands of years.
Revelations 1:1 The Revelation of Jesus Christ, which God gave unto him, to shew unto his servants things which must SHORTLY COME TO PASS.