^6:10 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?”

They called out in a loud voice,

These are those souls under the alter in verse 9 who were killed for the Word and their testimony.

“How long, Sovereign Lord,

This term, Soverign Lord,

• as in Master/Slave relationship that honors the Master (1 Timothy 6:1-2) and is submissive (1 Peter 2:18-20); this results in favor.
• as in God, the Great Creator (Acts 4:23-26).
• as in Jesus Christ who constantly is denied (2 Peter 2:1-3) even unto the "last days" (Jude 4).

holy and true,

This is the same description of Jesus given by the angel to the church in Philadelphia, Revelation 3:7.

until you judge the inhabitants of the earth and avenge 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.

• Romans 12:18-19
• Luke 18:1-8
• 2 Thessalonians 1:5-8-10

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!