Why did God kill the firstborn of Egypt?

Because Pharaoh refused to release God's firstborn.

In the Bible, the tenth plague is not where the firstborn theme starts. It is where it lands. Four chapters earlier, before any plague has fallen, God tells Moses exactly what to say to Pharaoh:

"And you shall say to Pharaoh: 'Thus says the LORD: Israel is my firstborn son. And I say to you: Let my son go, that he may serve me. If you refuse to let him go, behold, I will kill your son, your firstborn.'" — Exodus 4:22–23

The Hebrew word is בְּכוֹר (bekhor, H1060). It means "firstborn" — the one who, in ancient Israel, received the double portion of inheritance, the priestly standing, and the special consecration to the LORD (Exo 13:2; Deut 21:17). A family's firstborn wasn't just the oldest child. He was the one who carried the family's future.

And God says the nation of Israel is his firstborn. Collectively. The whole people is what every firstborn Israelite son would be to his family: the bearer of the inheritance, the consecrated one, the first.

Pharaoh's refusal to release Israel is not a property dispute or a labor dispute. It is theft of God's firstborn. The plagues escalate through the natural order — water, frogs, lice, flies, livestock, boils, hail, locusts, darkness — and then come to rest on the thing God named first: firstborn for firstborn.

Exodus 12:29 narrates it plainly: "At midnight the LORD struck down all the firstborn in the land of Egypt." Every Egyptian firstborn dies because Pharaoh held God's firstborn. The principle is one of substitution. The oppressor's firstborn dies so that God's firstborn goes free.

And Israel's firstborn are not struck — because they are covered. The blood of the Passover lamb marks every Israelite doorpost (Exo 12:7, 13). The firstborn is redeemed by substitution even within the camp of the redeemed. That pattern of substitutionary covering carries forward: every Israelite firstborn belongs to YHWH from that night forward (Exo 13:2), but the whole tribe of Levi absorbs the priestly obligation on behalf of all of them (Num 3:11–13). The first substitution (lamb for firstborn) is followed by a second (Levites for firstborn).

Second Temple Jewish tradition already read the two events together. Wisdom of Solomon 18:13 puts it exactly: "at the destruction of the firstborn, they acknowledged that the people were God's son."

The New Testament picks up the same logic. Christ is called God's πρωτότοκος — "firstborn" (G4416) — over creation (Col 1:15), from the dead (Col 1:18), among many brothers (Rom 8:29). The Passover lamb language binds on: Christ is "our Passover lamb, sacrificed for us" (1 Cor 5:7). The night the firstborn died in Egypt becomes the shape of what the cross later does: the substitutionary covering that lets God's firstborn go free.

For the full argument — including Psalm 89:27's declaration that David was made firstborn by divine appointment, and Hebrews 12:23's plural "firstborn ones" — see the study The Firstborn.