Midrash sa Exodo 11:5: Shemot Rabbah, Midrash Tanchuma, Mekhilta DeRabbi Yishmael & Midrash Tanchuma Buber

“I will harden his heart” – in order to administer their sentence.
“You shall say to Pharaoh: So said the Lord: My son, My firstborn, Israel” (Exodus 4:22).
“You shall say to Pharaoh: So said the Lord: My son…” – the Holy One blessed be He revealed to him that Pharaoh will not let Israel go until the plague of the firstborn; therefore, it was not necessary for the verse to inform him of it later.⁸When God said to Moses: “One more blow I will bring upon Pharaoh and upon Egypt; afterward he will send you forth from here” (Exodus 11:1), He did not specify what that blow would be. However, Moses knew that if it was the last blow it must be the plague of the firstborn, and he warned Pharaoh accordingly (Exodus 11:5). What is “My son, My firstborn, Israel”? It is a reference to Jacob their ancestor, who purchased the birthright from Esau so he would be able to serve God.⁹The firstborn were initially designated to perform sacrificial rites in service of God. Israel is another name for Jacob. You shall say to him: “Let My son go, and he will serve Me” (Exodus 4:23). He said to him: ‘If you prevent My firstborn from serving Me, I will prevent your firstborn from serving you, as I will kill them all.’

And it came to pass in the middle of the night, that the Lord smote all the firstborn in the land of Egypt (Exod. 12:29). The Creator of the night divided the night precisely. The Lord smote all the first born. It was the Lord Himself who smote the firstborn and not His emissary (Moses). In fact, even though a man was elsewhere, and his firstborn was in Egypt, he died. How do we know that the firstborn of the Cuthites, Puthites, and Ludites were also slain? It is said: And smote all the firstborn in Egypt, the first fruits of their strength in the tents of Ham (Ps. 78:51).⁸Ham was the ancestor of Cush, Put, and Lud (Gen. 10:15). Only the firstborn of the Pharaoh remained alive in fulfillment of the verse However, it was for this that I raised you up (Exod. 9:16). And Ba’al Saphon was the only idol remaining, to mislead them, in fulfillment of the verse He increaseth the nations, and destroyeth them (Job 12:23). Unto the firstborn of the captives (Exod. 12:29). Why were the firstborn of the captives punished? Because they had rejoiced in the decrees promulgated against Israel. Hence it is written: He that is glad at calamity shall not be unpunished (Prov. 17:5). You must not be of the opinion that only the captives reacted in that manner, for the slaves and handmaidens did likewise, as is said: Even unto the firstborn of the maidservants that is behind the mill (Exod. 11:5); that is, even those who were legally bound to the millers. Even their firstborn cattle were destroyed lest the people assert: “Our deities are powerful, and that is why punishment was imposed upon them (and not upon us).”

And we are hereby apprised that the captives rejoiced in all the decrees inflicted by Pharaoh upon Israel, (for which they were punished) in keeping with (Mishlei 17:5) "He who rejoices in (another's) misfortune will not be absolved," and (Psalms 24:17) "Do not rejoice in the downfall of your foe," and (Ezekiel 26:2-3) "Son of man, because Tyre said about (the besieged Jerusalem) 'Heach!' (an expression of joy) — thus said the L–rd G–d: 'Behold, Tyre, I am (coming) against you, and I will raise up many nations against you, as the sea raises its waves!'" And not only captives alone (were thus smitten), but even men-servants and maid-servants, viz. (Exodus 17:5) "… until the first-born of the maid-servant."