Parashas Masei · Passuk 35:27 · TT note 1
Chabad.org — English translation + Rashi
Torah Temimah points to Makkos 12a (TT note on “וְרָצַח גֹּאֵל הַדָּם”).
English: It was taught: Rabbi Akiva says, “and the blood-avenger shall kill the murderer” — it is optional for the blood-avenger, and no one else is liable on his account [if they kill him].
The avenger’s right is optional, not obligatory; others who kill the exile outside are not liable.
Only the blood-avenger has license to kill the exile who left the city; anyone else who does so is not subject to the same license (or, per the reading, is free of liability for the act).