TT #395 · Parashas Shoftim · Seventh Aliyah · Passuk 21:09
And you shall purge the innocent blood from your midst, when you do what is right in the eyes of the Eternal.
Torah Temimah points to Sifrei Devarim 210 / Sotah.
TT lemma: וְאַתָּה תְּבַעֵר הַדָּם הַנָּקִי.
Sefaria — Sifrei Devarim 210 / Sotah
English: ‘And you shall purge the innocent blood from your midst’ – it is a mitzvah to purge it.
Derash classifying the command (Sifrei) — the people are charged with actively removing the blood-guilt.
Yisrael is commanded to purge innocent blood from its midst by performing the rite and by doing what is right in Hashem’s eyes.