Sforno on Exodus 12:12/ספורנו על שמות י״ב:י״ב

1ועברתי, 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.]

2והכיתי כל בכור, “I, personally, seeing I can distinguish between the drops of semen which become a firstborn and those that do not.”

3ובכל 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.

4'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.