The Torah and the Sages teach that the Jews were spared the hardships of the plagues. For example, while the Egyptian was stricken with boils the Jew standing beside him was completely healthy. The final plague, however, presented a different scenario. God tells Moses to tell the Jews to place the blood of the slaughtered paschal lamb on the doorposts of their homes as a sign for Him to pass over them when smiting the first born of Egypt. It follows that those Jews who chose not to place this sign upon their doors were smitten along with the Egyptians. Why was this final plague different than the first nine where the Jews were spared from the fate of the Egyptians? Why was blood chosen to be the sign that would save the Jews from the fate of the Egyptians?
The Torah teaches that God unleashed his complete destructive power (Mash-cheet) against the Egyptians in the final plague. This terminology is not used regarding the other plagues possibly because the other plagues were intended not to kill the Egyptians, but to make them accept God’s supremacy. The Jews already accepted and believed in God so they didn’t need to experience the plagues. The final plague was no longer to teach. Rather it was to destroy and punish. When God’s attribute of strict judgment is unleashed it makes no exceptions. All are judged equally. There is no mercy.
The Sages teach that the Jews were on the forty ninth level of impurity in Egypt, with fifty being the absolute lowest. If they had reached fifty they would not have been redeemed. At the splitting of the sea the angels asked God why he would choose to save the Jews and kill the Egyptians, since both nations worshipped idols and were not much different. Therefore, it is clear that were they to be judged with the attribute of strict judgment the Jews might very well be liable to suffer the same punishment as the Egyptians.
Why was blood chosen as the sign to save them? The Sages teach that the original cause of the Egyptian bondage was the “lashon hara” or “evil speech” that Joseph spoke against his brothers, which began the chain of events that brought the Jews to Egypt. They also teach that the Jews
were redeemed from Egypt because they did not speak Lashon Hara. The Midrash says that the Jews didn’t change their language to that of the Egyptians meaning, I think, that they didn’t speak Lashon Hara.
The Hebrew word for blood is “Daam” which is also the root of the Hebrew word for “silence”. Therefore, the blood that the Jews placed on their doorposts represented their merit that differentiated them from the Egyptians: they didn’t speak Lashon Hara.