Or HaChaim on Exodus 11:4/אור החיים על שמות י״א:ד׳

1 כחצות הלילה, "at around midnight, etc." The Talmud Berachot 4 already discusses that the reason that Moses did not give the precise time but only an approximation was so that in the event the Egyptian astronomers were inaccurate in their calculations they should not be able to accuse Moses of having lied. Possibly the reason is related to what Bereshit Rabbah 43,3 has to say about Genesis 14,15 where Abraham is described as defeating the four kings. That event occurred on the night of the 15th of Nissan. In that case the letter כ before the word חצות does not describe an approximation but refers to a previous event which took place on that date at that time. Our sages in Shemot Rabbah 42 tell us that the night in which Abraham defeated the four mightiest kings of the world (Genesis 14,15) caused many people to re-evaluate the beliefs they had held up to that time. The Torah therefore mentioned the time when this miracle had occurred. In our paragraph too, the people who survived this event must have done some serious soul-searching as to the validity of their religious beliefs. The letter כ then refers to such re-evaluations as a result of what would occur at midnight.

2אני יוצא. "I will go out, etc." G'd stressed the "I" as if to say that although He has many agents who could perform this task at His behest, He would do so personally and would descend right into the midst of Egypt. G'd had two reasons for doing this personally. 1) As an expression of His fondness for His people whom He wanted to liberate personally. 2) Angels are not capable of distinguishing which human being was formed from the first drop of semen of his father so that he deserved the description בכור, firstborn. Even if such a fetus would emerge as the second or third of a multiple birth it would still deserve to be considered the firstborn. No one but G'd Himself is able to make these distinctions.