Revelation 6:9-11:A cry for justice.
9 When he opened the fifth seal, I saw under the altar the souls of those who had been slain because of the word of God and the testimony they had maintained. 10 They called out in a loud voice, ‘How long, Sovereign Lord, holy and true, until you judge the inhabitants of the earth and avenge our blood?’ 11 Then each of them was given a white robe, and they were told to wait a little longer, until the full number of their fellow servants, their brothers and sisters, were killed just as they had been.”NIV
It’s important to note that the souls of the martyrs, under the altar, are not calling out for vengeance, but for justice. It’s not that they want to get even. They appeal to God’s character: ‘’Sovereign Lord, holy and true…’’ (10). If God is who He is, He must punish those who have taken their lives. They have been treated unjustly, as if they were criminals. It’s not fair. So it’s a question of when. ‘’How long…?’’ (See Psalm 94:3; Hab.1:2).
Ultimately, this is a longing for the honour of God’s Name. It’s zeal for His reputation. Unless we see this we will fail to understand the prayer. If God is holy and just, He must punish unrepentant sinners.