Rashi on Exodus 11:5/רש"י על שמות י״א:ה׳

1עד בכור השבי UNTO THE FIRSTBORN OF THE CAPTIVE (these words are used in 12:29 instead of עד בכור השפחה “unto the firstborn of the handmaid” in this verse) — Why were the captives smitten, for they had not enslaved the Israelites? In order that these might not say that their god had claimed satisfaction for the humiliation imposed upon them and had brought this punishment upon the Egyptians (cf. Mekhilta).

2מבכור פרעה עד בכור השפחה FROM THE FIRSTBORN OF PHARAOH UNTO THE FIRSTBORN OF THE HANDMAID — All who were of less importance than the firstborn of Pharaoh and of more importance than the firstborn of the handmaid are included in this description. And why were the sons of the handmaids stricken? Because they, too, treated them (the Israelites) as slaves and rejoiced at their misery (Midrash Tanchuma, Bo 7).

3וכל בכור בהמה AND ALL THE FIRSTBORN OF BEASTS — because they worshipped these as gods. When the Holy One, blessed be He, exacts punishment from a nation, He punishes their gods at the same time (Genesis Rabbah 96:5).