"Around the stroke of midnight, I will go out in Egypt." (Ex. 11:4)

When G‑d dispatches an agent to kill those whom He wants to kill, such an agent, i.e. an angel, does not have the ability to distinguish between the righteous and the wicked. Also, the Zohar says that whenever there is a punitive activity, even by G‑d directly, additional destructive forces become active also. Thirdly, when a person is guilty of some misdemeanor, these forces of destruction attach themselves to him and whenever someone else is being punished in the vicinity of such a guilt-ridden person, that person is also liable to become a victim at such a time.

The Angel of Death… would not be able to distinguish between the just and the unjust…

G‑d reasoned that, even assuming that the Israelites who had survived the plague of darkness were all righteous, He could not assure that all of them would take part in the Exodus to follow if He left the slaying of the firstborn to an angel, i.e. the Angel of Death. The latter would not be able to distinguish between the just and the unjust. Seeing that not all the Israelites were righteous, such distinction between the Jewish firstborn and the Egyptian firstborn would be even more difficult to make. G‑d therefore had to descend personally to carry out this plague in order to save the Jewish firstborn.

This was especially so since the spiritual power representing Egypt in the celestial regions pointed to the fact that both Jews and Egyptians were worshipping idols (see Yalkut Reuveni Beshalach).

Due to all these considerations, as well as in order to counter the argument by the spiritual agent of Egypt, G‑d commanded the Jewish people to demonstrate that they denied the Egyptian deities by risking death and slaughtering the Egyptian deity, the lamb.

[Translation and commentary by Eliyahu Munk]