Kommentar zu Schemot 12:13

והיה הדם לכם לאת AND THE BLOOD SHALL BE UNTO YOU FOR A SIGN — it shall be to you for a sign, and not to others for a sign (Mekhilta d'Rabbi Yishmael 12:13:1). From this we may learn that they put the blood only inside their houses.

ולא יהיה בכם נגף למשחית, so you will not be struck with a plague as a corollary to the retributive action I am taking against Egypt.

ולא יהיה בכם נגף למשחית, "so there will be no plague against you to destroy, etc." What is meant is that the destructive force (angel of death) will have no excuse to attack you rather than his other natural targets; he will notice the blood on the doorposts of your homes as if it were the seal of the Almighty and be afraid to approach and inflict harm. This was the reason G'd commanded the Israelites to use the blood as a sign. Should you say that in that case any Egyptian hiding out in the house of an Israelite which bore the sign of that blood could escape the plague, the Torah wrote בכם, the restrictions on the angel of death applied only "to you." The "light" of having performed the commandment illuminated only Israel; we find confirmation of this in Isaiah 60,2: ועליך יזרח ה וכבודו עליך יראה, "but upon you the Lord will shine and His Presence will be seen over you."