"And for the resident alien and for the settler." Or just as Israel is exiled through the actions of an Israelite, so too Israel is exiled through the actions of a resident alien. You may say, if when an Israelite killed him, he is exempt, a fortiori, he should not be exiled, but rather, if he killed an Israelite, he is put to death. These six cities shall be for refuge. Why is it stated? Because it says, "You shall give three cities on the other side of the Jordan" (Deuteronomy 19:7). I might have thought that I should first designate the first city as a city of refuge; therefore, the verse states, "These six cities shall be." This teaches that not one of them shall be designated until all six are designated.
Exile Falls on the High Priest for Collective Sin
Yalkut Shimoni on Torah 787:19
וְלַגֵּר וְלַתּוֹשָׁב. אוֹ כְּשֵׁם שֶׁגּוֹלֶה עַל יְדֵי יִשְׂרָאֵל, כָּךְ יִשְׂרָאֵל גּוֹלֶה עַל יָדוֹ. אָמַרְתָּ, אִם כְּשֶׁהֲרָגוֹ יִשְׂרָאֵל פָּטוּר, קַל וָחֹמֶר שֶׁלֹּא יִגְלֶה, אֶלָּא אִם הֲרָגוֹ יִשְׂרָאֵל הָרוּג, וְאִם הָרַג אֶת יִשְׂרָאֵל נֶהֱרָג. תִּהְיֶינָה שֵׁשׁ הֶעָרִים הָאֵלֶּה לְמִקְלָט, לָמָּה נֶאֱמַר. לְפִי שֶׁהוּא אוֹמֵר "אֵת שְׁלֹשׁ הֶעָרִים תִּתְּנוּ מֵעֵבֶר לַיַּרְדֵּן" שׁוֹמְעַנִי אֶת הָרִאשׁוֹנָה שֶׁמַּפְרִישִׁין תְּהֵא קוֹלֶטֶת, תַּלְמוּד לוֹמַר "תִּהְיֶינָה שֵׁשׁ הֶעָרִים הָאֵלֶּה", מַגִּיד שֶׁלֹּא קָלְטָה אַחַת מֵהֶן עַד שֶׁהֻפְרְשׁוּ כֻּלָּן.