Adam Clarke on Revelation 6:10

And they cried with a loud voice - That is, their blood, like that of Abel, cried for vengeance; for we are not to suppose that there was any thing like a vindictive spirit in those happy and holy souls who had shed their blood for the testimony of Jesus. We sometimes say Blood cries for blood; that is, in the order of Divine justice, every murderer, and every murdering persecutor, shall be punished.

O Lord - Ὁ Δεσποτης· Sovereign Lord, supreme Ruler; one having and exercising unlimited and uncontrolled authority.

Holy - In thy own nature, hating iniquity;

And true - In all thy promises and threatenings;

Dost thou not judge - The persecutors;

And avenge our blood - Inflict signal punishment;

On them that dwell on the earth? - Probably meaning the persecuting Jews; they dwelt επι της γης, upon that land, a form of speech by which Judea is often signified in the New Testament.