Le Deutéronome 19:12 Midrash : Sifrei Devarim

וְשָֽׁלְחוּ֙ זִקְנֵ֣י עִיר֔וֹ וְלָקְח֥וּ אֹת֖וֹ מִשָּׁ֑ם וְנָתְנ֣וּ אֹת֗וֹ בְּיַ֛ד גֹּאֵ֥ל הַדָּ֖ם וָמֵֽת׃

les anciens de sa ville le feront extraire de là et le livreront au vengeur du sang pour qu’il meure.

Sifrei Devarim

One who was liable to the death penalty was killed, viz. "Then the elders of his city shall send, and they shall take him from there."

Sifrei Devarim

(Devarim, Ibid. 12) "and they shall place him into the hand of the avenger of blood and he shall die": Whence is it derived that if he did not die at the hands of the avenger of blood, he is to die at the hand of any man? From "and he shall die."