Why idolaters are called "own brothers" and "own fellow Jews"?

Question

I was taught that all positive interpersonal Mitzvos are phrased in the Torah with "אחיך" or "רעך" (brother of fellow Jew) to signify that they only apply to Kosher pious Jews, for example, returning lost property or loving thy neighbor (e.g. see MT Mourning.14.1: "עֲשֵׂה אַתָּה אוֹתָן לְאָחִיךְ בְּתוֹרָה וּבְמִצְוֹת:")

However, when God orders the execution of Golden Calf worshippers, He specifically calls them "brothers" and "fellows":

"וַיֹּאמֶר לָהֶם כֹּה־אָמַר יְהוָה אֱלֹהֵי יִשְׂרָאֵל שִׂימוּ אִישׁ־חַרְבּוֹ עַל־יְרֵכוֹ עִבְרוּ וָשׁוּבוּ מִשַּׁעַר לָשַׁעַר בַּמַּחֲנֶה וְהִרְגוּ אִישׁ־אֶת־אָחִיו וְאִישׁ אֶת־רֵעֵהוּ וְאִישׁ אֶת־קְרֹבוֹ׃"
He said to them, “Thus says the LORD, the God of Israel: Each of you put sword on thigh, go back and forth from gate to gate throughout the camp, and [a person should] slay [his own] brother, neighbor, and kin.” -Exodus 32:27

The Torah could say instead "slay all wrongdoers from your kin" or "your wicked relatives".

Why the wicked are called "own brother" and "own fellow" here?

Answer 1

Because even if it was their own brother they had to be killed. A person might think that if it was their brother or good friend they may have mercy and spare them, therefore, it expressly writes 'brother' and 'fellow' to highlight the need for them all to be killed.

The Ibn Ezra writes:

והרגו איש את אחיו. אפי' יהיה אחיו. ורבים פירשו האומר לאביו ולאמו על זה המעשה והוצרכו לפרש אביו ואמו אבי האם ואין צורך כאשר אפרש במקומו:

AND SLAY EVERY MAN HIS BROTHER. Even if it be his brother. Many interpret Who said of his father, and of his mother: I have not seen him (Deut. 33:9) as referring to this incident. They had to explain his father and his mother as referring to the father of the mother. However, as I will explain, there is no need for this interpretation. (Sefaria translation)

And Ramban adds:

והרגו איש את אחיו שלא תחמלו ולא תכסו על אח וריע וקרוב

AND SLAY EVERY MAN HIS BROTHER. This means that they were not to spare nor conceal brother, friend, or relative.

An alternative approach is adopted by Rashi, who reads it more as a technicality, in that the term 'brother' here must imply that they share the same mother which would mean that they are not a Levi but a Yisrael instead, as all the Leviim were righteous. (Refer to Masechta Yoma 66b and also to the Sifsei Chachomim for his explanation on Rashi).