“[…] I saw underneath the altar the souls of those slaughtered […]. A white robe was given to each of them, and they were told to rest a little while longer, until the number was filled of their fellow slaves and their brothers who were about to be killed as they had been.”
~Revelation 6:9,11

After the four horsemen account, this passage says there are “souls” shouting out for justice. (Rev. 6:10)
God’s Word explains that someone’s soul, or person, is in their blood. (Lev. 17:11)
The imagery of a person’s blood calling for justice is not new. (Gen. 4:10)
And Christians who died before Christ’s second coming were actually dead and not lingering in some sort of afterlife. (1 Thess. 4:15,16)
If the “souls” shouting out for justice are representative of Christians’ shed blood, in what sense are they given white robes?
This chapter begins by dynamically describing Christ’s coronation and subsequent catastrophic events on earth. (Rev. 6:2-8)
Between then and the ‘number of fellow slaves being filled,’ there was a moment in which dead Christians who were meant to rule with Christ in heaven were raised and each granted a white robe.
They have been resurrected to immortality and the white robes symbolize their righteous acts. (Rev. 3:5; 19:8)
They “rest” in the sense that they must patiently wait for God’s judgment day before being allowed to avenge their deaths. (Rev. 7:3,4; Rev. 17:14)
So while initially the word “souls” represents the shed blood of loyal Christians, halfway through the passage it is referring to their resurrected selves.