All the water in Mitzrayim turned into blood. Blood symbolizes: פִ שְ יכוּת דָ מִ ים bloodshed (like Paroh told Moshe: רְ אוּ כִ י רָ עָ ה נֶגֶד פְ נֵיכֶם י י “Behold, the star of blood hangs over you” since the astronomers saw blood and killing in the sky. Rashi). Paroh used Jewish blood to bathe; now let him drink blood. Notice how in the end, the blood of the Korban Pesach (on the: שְ מַ קוֹף וּמְ זוּתזוֹ three sides of the doorframe) saved Klal Yisroel from the: חִ שְ מַ ית angels of destruction. Since when does the symbol of evil and death become a Zechus?
And the same in the Bais Hamikdash, דָ ם blood was always used on the Mizbeach to be M'chaper. The truth is, that our evil side is our best Zechus, since we overcome it and we do Teshuva. Like Dovid who had to be an Admoni, and did only good despite his evil nature. A Malach doesn’t receive the Torah, as Moshe told them: קִ נְ אָ ה יֵש בֵ ינֵיכֶם? שבת פח ב You Malachim have no bad Middos, so the Torah isn’t for you. (Maharal). No Yetzer Hara, no Torah; and no credit for being pure without sin.
On Yom Kippur when we wear a white Kittel and fast like Malachim, we also read the Parsha of Arayos. Why? To show our true greatness, since we need to be warned against such lowlife Arayos. This shows what we are capable of doing, and our lofty Yom Kippur fasting mode is much more appreciated.
Yitzchok Avinu was a holy Olah Temima (pure sacrifice to Hashem). Yet, before he gave the Brachos, he ate: מַ טְ עַ מִ ים כַאֲ שֶ ר אָ הֵ ב תודלות כז יד dishes according to his taste. The fact that he is a normal human makes his high Madrega truly high; he has to overcome and rise, despite his Gashmiyus'dik side. And: שַ בָ ת הַ יוֹם לַ ה' the holy Shabbos should really be total immersion in Torah and closeness to Hashem. Yet, you are not “Yotzei” Shmiras Shabbos without tasty dishes and Oneg Shabbos.
The Ikar is to show that you are a normal human, and yet you rise and live a life of Ruchaniyus. RSRH Zatzal says that the reason why the family background of Moshe is mentioned, is to show that he was a normal human, and anybody can become great like Moshe. And even in one day, a “nobody” can become a Gadol.
