Ramban explains that the strike against Egypt's firstborn shows God's direct rule over beings, households, and the heavenly powers that Egyptians worshiped. The blood of Passover marks Israel as exempt from the plague, highlighting the idea that miracles can be selective, covenantal, and intertwined with recognizable physical signs.
אֶעֱשֶׂה שְׁפָטִים אֲנִי ה' אֲנִי בְּעַצְמִי וְלֹא עַל יְדֵי הַשָּׁלִיחַ. זֶה לְשׁוֹן רַשִׁ"י (רש"י על שמות י"ב:י"ב). וּמִדְרַשׁ חֲכָמִים אֵינוֹ כֵן, אֶלָּא וְעָבַרְתִּי בְּאֶרֶץ מִצְרַיִם, אֲנִי וְלֹא מַלְאָךְ וְהִכֵּיתִי כָל בְּכוֹר, אֲנִי וְלֹא שָׂרָף, וּבְכָל אֱלֹהֵי מִצְרַיִם אֶעֱשֶׂה שְׁפָטִים, אֲנִי וְלֹא הַשָּׁלִיחַ, אֲנִי ה', אֲנִי הוּא וְלֹא אַחֵר. וּפֵרוּשׁוֹ, כִּי בַּעֲבוּר שֶׁהַפָּרָשָׁה דִּבּוּר מֹשֶׁה עִם יִשְׂרָאֵל, הָיָה רָאוּי שֶׁיֹּאמַר "וְעָבַר ה' בְּאֶרֶץ מִצְרַיִם, וְהִכָּה כָל בְּכוֹר", וּלְכָךְ יִדְרְשׁוּ כִּי וְעָבַרְתִּי אֲנִי בְּעַצְמִי, לֹא עַל יְדֵי מַלְאָךְ שָׁלוּחַ מֵאִתּוֹ בַּמַּגֵּפוֹת, כָּעִנְיָן בְּדָוִד (שמואל ב כ"ד:ט"ז) וּבְסַנְחֵרִיב (מלכים ב י"ט:ל"ה). וְהִכֵּיתִי כָל בְּכוֹר, אֲנִי וְלֹא שָׂרָף, לוֹמַר כִּי גַּם הַמַּכָּה תִּהְיֶה עַל יַד הקב"ה, לֹא כְּמֶלֶךְ שֶׁיַּעֲשֶׂה נְקָמָה בְּשׂוֹנְאָיו לְפָנָיו עַל יְדֵי סְפַּקְלְטוֹרִים שֶׁלּוֹ, שֶׁהֵם הַשְּׂרָפִים שֶׁמֵּהֶם תֵּצֵא אֵשׁ שׂוֹרֶפֶת בְּאוֹיְבָיו, כְּעִנְיַן אֵלִיָּהוּ בְּשָׂרֵי הַחֲמִשִּׁים: וּבְכָל אֱלֹהֵי מִצְרַיִם אֶעֱשֶׂה שְׁפָטִים וְלֹא עַל יְדֵי הַשָּׁלִיחַ הַשָּׁלוּחַ מֵאִתּוֹ יִתְבָּרַךְ לְכָל הַמַּעֲשִׂים הַנַּעֲשִׂים בָּאָרֶץ, וְהוּא הַמַּלְאָךְ הַגָּדוֹל שֶׁנִּקְרָא בַּעֲבוּר כֵּן מְטַטְרוֹן. כִּי פֵּרוּשׁ הַמִּלָּה מוֹרֵה הַדֶּרֶךְ, כְּמוֹ שֶׁאָמַר בְּסִפְרֵי (האזינו שלח), אֶצְבָּעוֹ שֶׁל הקב"ה נַעֲשָׂה מְטַטְרוֹן לְמֹשֶׁה וְהֶרְאָהוּ כָּל אֶרֶץ יִשְׂרָאֵל. וּבִילַמְּדֵנוּ (מובא בערוך ערך מטטר), "וַיִּשְׁמַע בָּלָק כִּי בָא בִלְעָם" (במדבר כב לו), שֶׁשָּׁלַח מְטַטְרוֹן לְפָנָיו. וְעוֹד שָׁם "רְאֵה הַחִלֹּתִי תֵּת לְפָנֶיךָ" (דברים ב לא), אִי אִכְפַּת לְךָ אֲנִי מְטַטְרוֹן שֶׁלְּךָ. וְאַל תִּתְמַהּ, שֶׁהֲרֵי אֲנִי עָתִיד לֵעָשׂוֹת מְטַטוֹר לִפְנֵי אָדָם עָרֵל, לִפְנֵי כּוֹרֶשׁ, שֶׁנֶּאֱמַר "אֲנִי לְפָנֶיךָ אֵלֵךְ" (ישעיה מה ב), לִפְנֵי אִשָּׁה אֲנִי עָתִיד לְקַדֵּם, לִפְנֵי דְּבוֹרָה וּבָרָק, שֶׁנֶּאֱמַר "הֲלֹא ה' יָצָא לְפָנֶיךָ" (שופטים ד יד), וְכֵן בִּמְקוֹמוֹת רַבִּים. וְכֵן שָׁמַעְתִּי כִּי שָׁלִיחַ בִּלְשׁוֹן יָוָן "מְטַטָּר". וְדָרְשׁוּ אֲנִי ה', אֲנִי הוּא וְלֹא אַחֵר, שֶׁהוּא אֶחָד וְאֵין אֱלֹהִים עִמּוֹ לִמְחוֹת עַל יָדוֹ. וְזֶהוּ עִנְיַן הַמִּדְרָשׁ הַזֶּה:
I WILL EXECUTE JUDGMENTS: I AM THE ETERNAL. “I Myself and not by means of the messenger.” Thus the language of Rashi. But the Midrash of the Sages is not so. Instead, the Midrash reads: “And I will go through the land of Egypt — I, and not an angel. And I will smite all the firstborn — I, and not a seraph. And against all the gods of Egypt I will execute judgments — I, and not the messenger. I am the Eternal — I am He, and no other.” The purport of the Midrash is as follows: Since this chapter contains the words of Moses to Israel, it would have been proper for the verse here to say, “And the Eternal will go through the land of Egypt and He will smite all the firstborn,” [instead of saying, And I will go through]. It is for this reason that the Rabbis interpreted the expression, and I will go through the land of Egypt, as meaning “I Myself” and not a messenger sent by Him in plagues, as in the days of David and Sennacherib. “And I will smite all the firstborn — I, and not a seraph,” means that the striking of the plague will be done by the Holy One, blessed be He, and not like a king who wreaks vengeance on his enemies through his executioners, their counterparts on high being the seraphim from whom the fire comes forth consuming His enemies, as in the case of Elijah and the captain of the fifty. “And against all the gods of Egypt I will execute judgments, and not by means of the messenger” sent by Him, blessed be He, for whatever is to be done upon the earth. This is the great angel, who on account of it is called Mattatron, the meaning of the word being “the guide of the road.” Thus the Rabbis have said in the Sifre: “The Holy One, blessed be He, was the mattatron (guide) for Moses, and He showed him the entire Land of Israel.” And in the [Midrash] Yelamdeinu we find: “And Balak heard that Balaam came for Balaam sent his mattatron before him.” And it is furthermore written there: “Behold, I have begun to deliver up Sihon and his land before thee. Do not let it worry you. I am your mattatron. And be not surprised at this for I am even to be the mattatron before an uncircumcised person, i.e., before Cyrus [king of Persia], as it is said, I will go before thee. Before a woman — Deborah and Barak — I will lead on the way, as it is said, Is not the Eternal gone out before thee.” [We find this word mattatron] also in many other places. I have also heard that in the Greek language, a messenger is called mattator. And [finally] the Rabbis interpreted [in the Midrash mentioned at the beginning of this discussion]: “I am the Eternal — I am He, and no other,” meaning that He is one, and there is no other god beside Him to protest against His act. This is the purport of this Midrash.