Moshe said, “So said Hashem, At the dividing point of the night, I will go out into the midst of Egypt (11:4)
On the possuk “And I will pass through the land of Egypt” (12:12)) Chazal expound that Hashem said “I and not an angel”.
The Zohar (Medrash Ne’elam, Vayero) asks two questions. Firstly. why did the Egyptians receive the seeming honor of being punished by Hashem Himself rather than by a mere angel? Secondly, if the camp of the Assyrians was smitten by an angel (see Yeshaya 37:36) all the more so should the Egyptians have received retribution through the medium of an angel, because the Egyptian nation was the most degenerate and immoral nation of all, and an angel would seem to be the best medium for punishing them rather than Hashem Himself.
The Zohar answers that specifically because they were so depraved and steeped in tumah was it not appropriate to send something as holy as an angel into their midst. Hence, this was a sign of disgrace for the Egyptians rather than a sign of honor.
So we see that the tumah of Egypt with its heresy and immorality was so severe that even angels were not to come into contact with it. This teaches us how foolish those are who go to live amongst irreligious or non-Jewish people whose way of life is very far removed from their own, thinking that they will surely not be harmed by this. If Hashem wants holy angels to stay away from places of tumah, how much more so mere mortals, who are surely even more likely to be harmed by their surroundings.
Rabbi Sternbuch’s comments on Parshas Bo – the English Parsha sheet