The Mekhilta, the tannaitic midrash on Exodus, asks a devastating question about the plague of the firstborn. The verse says God struck down "until the captive firstborn" — including prisoners locked in Egypt's dungeons (Exodus 12:29). But what did the captives do wrong? They were already suffering. They were enslaved just as the Israelites were. Why kill their firstborn too?
The Mekhilta's answer reveals a concern that goes beyond punishment. The captives were killed not for their own sins, but to prevent them from making a theological claim after the fact. If the captive firstborn survived while Egyptian firstborn died, the captive nations would have said: "Our god brought this catastrophe upon Egypt for imprisoning us. Our god is awesome — he stood up for himself and shielded us from this punishment!"
In other words, the surviving captives would have attributed the plague to their own deities. They would have turned God's act of liberation into evidence for idolatry. The entire theological message of the Exodus — that there is one God who acts in history to free Israel — would have been confused by competing claims.
God could not allow rival interpretations. The tenth plague had to be so total, so comprehensive, that no nation could claim its gods had provided protection. Every firstborn died — Egyptian, captive, foreign — so that only one explanation remained: the God of Israel alone controls life and death.