6:10 They cried out in a loud voice, “Holy and true Master, how long before you judge and take revenge on those living on earth who shed our blood?”
They cried out in a loud voice,
These are those souls under the alter in verse 9 who were killed for the Word and their testimony.
“Holy and true Master,
This is the same description of Jesus given by the angel to the church in Philadelphia, Revelation 3:7.
how long before you judge and take revenge on those living on earth who shed our blood?”
This is not earthly hateful revenge. These souls are in heaven, pure and bought, BUT God's righteousness demands evil to be judged.
The curious thing about this portion is that, even in heaven, the souls do NOT know when God will perform this vengence. This is to say, both heaven occupants and earth occupants do NOT know when these seals will be broken. We do know, however, that God's timing is always perfect!