Rashi on Exodus 12:29/רש"י על שמות י״ב:כ״ט

1וה׳ “AND” THE LORD — Wherever it is stated וה׳ “And the Lord” it signifies He and His celestial Court, because the prefix ו expresses an addition, just as one says, “Mr. So-and-so and Mr. So-and-so” (Genesis Rabbah 51:2; cf. Rashi on Genesis 19:24).

2הכה כל בכור SMOTE ALL THE FIRSTBORN — all: such, also, belonging to another people who happened to be in Egypt (Mekhilta; cf. Rashi on v. 12).

3מבכר פרעה means FROM THE FIRSTBORN PHARAOH’S — Pharaoh too, was a firstborn, but he alone was allowed to remain of the firstborn; and regarding this it states, (Exodus 9:16) “[But for this cause I have maintained thee in life in order to show thee my power” — i. e. to show thee my power at the Read Sea (cf. Mekhilta).

4עד בכור השבי UNTO THE FIRSTBORN OF THE CAPTIVE — because they rejoiced at the misfortune of the Israelites (Midrash Tanchuma, Bo 7); and a further reason why they were slain is that they should not say, — if they remained alive — it was our god who brought punishment upon our oppressors, the Egyptians (Mekhilta) The firstborn of the handmaid, threatened with death in 40:5 but not mentioned here as having been slain, is included in those stated here to have been slain since it enumerates here (i. e. the terms used here are intended to include everyone) from the most important amongst all of them to the least important, and the firstborn of the handmaid as belonging to the Egyptian people, is certainly more important than the firstborn of the captive (cf. Rashi on 11:6).