Oppressing the Oppresser

This parallelism in Joel 3:19-21 illustrates the principle at work in Deuteronomy 19:19, which says regarding a false accuser, "You shall do to him as he had meant to do to his brother. So you shall purge the evil from your midst."

Egypt and Edom attacked Judah and Jerusalem without cause, but God turned their violence back on them and multiplied it.

Egypt and Edom	Judah and Jerusalem
V19 – Egypt shall become a desolation	V20 – Judah shall be inhabited forever
…and Edom a desolate wilderness	…and Jerusalem to all generations
…for violence done to Judah	V21 – I will avenge their blood
…because they shed innocent blood	…because YHWH dwells in Zion

Lest Egypt's punishment seem overly harsh, remember who created whom. Who is the potter and who is the clay?