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?