The Mekhilta, the tannaitic midrash on Exodus, extends its devastating logic about the plague of the firstborn to the animal kingdom. The verse states that God struck "every firstborn of the beast" (Exodus 12:29). The rabbis immediately ask the same question they asked about the captives: what sin did the beasts commit? Animals have no moral agency. They cannot rebel against God or oppress Israel. Why were their firstborn killed?
The Mekhilta's answer is identical in structure to its explanation for the captive firstborn. The beasts were killed so that the Egyptians could not make a claim about their animal gods. The Egyptians worshipped animals — bulls, rams, cats, crocodiles. These creatures were not merely livestock. They were deities.
If the firstborn of the beasts had survived the plague, the Egyptians would have said: "Our god brought this catastrophe upon us. But look — our god is awesome! The plague did not prevail against our god!" The survival of the sacred animals would have been interpreted as proof that the Egyptian deities had power to resist the God of Israel.
God destroyed the firstborn of every beast to demolish that argument before it could be made. Every object of Egyptian worship had to fall. The plague was not merely punishment for the Egyptians. It was a systematic dismantling of the entire Egyptian theological system — a judgment against every god of Egypt, as the Torah explicitly states (Exodus 12:12).