The martyrs called out to God because they trusted his power to address their grievances.
How long: Is God slow to act? Will justice be served? God acts decisively (see Revelation 6:12–14). His wrath (Revelation 6:16–17) should be understood as justice, fairness, and righteousness (see Exodus 34:5–7; Nehemiah 9:17; Psalms 103:8; 145:8; Joel 2:13; Jonah 4:2; Romans 2:6–11).
avenge our blood: This plea for God’s justice follows the pattern of psalms calling for vengeance (see Psalms 6:3; 74:10; 79:5; 80:4; 137:1–9). It also reflects the covenant curses of Deuteronomy 28:53–57; 32:35 (quoted in Romans 12:19).