Parashas Masei · Passuk 35:19 · TT note 2
Chabad.org — English translation + Rashi
Torah Temimah points to Sanhedrin 45b (TT note on “בְּפִגְעוֹ בּוֹ”).
E-Daf — Sanhedrin 45b · Sefaria
English: It was taught: “The blood-avenger shall put the murderer to death” — it is a mitzvah for the blood-avenger [to do so]. And from where [do we know] that if he has no [family] avenger the court appoints an avenger for him? From “when he encounters him” — in any case.
Halakhic appointment of a court-designated avenger when no family member exists.
Executing the intentional murderer is a mitzvah of the blood-avenger; if none exists, the court itself supplies one so that the killer does not go free.