Exodus 12:12

And I will pass. Like a king who passes from place to place. In a single pass and in a single moment they were all smitten.

Every first-born in the land of Egypt. Even other firstborns and they are in Egypt. And from where [do we learn that it applied to] Egyptian firstborns in other places? It says (Psalms 136:10) "Who strikes Egypt in their firstborns".

From man until beast. He who started the sin, from him the punishment began.

And to all the gods of Egypt. Of wood, they would rot. Of metal, they would melt and be poured to the earth.

I will judge, I am Hashem. I myself, and not through a messenger.

And I will be revealed in the land of Mizraim in the majesty of My glory this night, and with Me ninety thousand myriads of destroying angels; and I will slay all the firstborn in the land of Mizraim, of man and of beast, and against all the idols of the Mizraee I will execute four judgments: the molten idols shall be melted, the idols of stone be broken, the idols of clay shall he shattered, and the idols of wood be made dust, that the Mizraee may know that I am the Lord.

AND AGAINST ALL THE GODS OF EGYPT WILL I EXECUTE JUDGMENTS. The idol of wood rotted, and the one of metal melted.
Now Scripture did not elucidate the nature of these judgments because the vanities by which they are instructed are but timber. Similarly, at the time when this actually took place, it is written, And the Eternal smote all the firstborn … and all the firstborn of cattle, but does not mention the judgments executed against their gods. Their apprehension [on that night] concerned the death of the firstborn, as it is said, And Pharaoh rose up in the night, he, and all his servants, and all the Egyptians,, Verse 30. The verse there clearly states that “the great cry” was due to the death of the firstborn. whereas the judgments executed against their idols did not come to be known till the morning when they went to the house of their abominations. And it is written, And the Egyptians were burying them that the Eternal had smitten among them, even all their firstborn, and upon their gods the Eternal executed judgments.
In my opinion, Scripture alludes here to the lords on high, the gods of Egypt, something like the verse, The Eternal will punish the host of the high heavens on high, and the kings of the earth upon the earth. Thus He subdued the power of the Egyptians and that of the lords over them. But Scripture hints and deals briefly with hidden matters.

אעשה שפטים. This was because the Egyptians had put their trust in these deities; G’d had to demonstrate their utter helplessness to help themselves, much less those who worshipped them. It was an indirect response to Pharaoh’s insolence in asking “Who is Hashem, etc.?” (5,2)

ועברתי, in the sense of Psalms 78,50 יפלס נתיב לאפו, “He cleared a path for His anger;” something an agent, i.e. angel could not do. [once the agent has been given permission to wreak havoc, he cannot distinguish between the innocent and the guilty in that domain. Ed.]

והכיתי כל בכור, “I, personally, seeing I can distinguish between the drops of semen which become a firstborn and those that do not.”

ובכל אלוהי מצרים אעשה שפטים, I will humble the celestial representatives of the nation of Egypt. Once I have neutralised them, I can apply My retribution to their protégées with increased force.

'אני ה. This is something reserved for G’d personally, exclusively. The author of the Haggadah shel Pessach paraphrases this by writing אני הוא ולא אחר, “I alone and no one else.” This expression explains all that has been written before, i.e. why in this instance G’d personally had to mete out the retribution as opposed to the angel who killed 185,000 men of Sancheriv’s army all in the same night without assistance. (Kings II 19,35) There the angel did not have to divine who was a firstborn and who was not.