פֶּן־יִרְדֹּף֩ גֹּאֵ֨ל הַדָּ֜ם אַחֲרֵ֣י הָרֹצֵ֗חַ כִּי־יֵחַם֮ לְבָבוֹ֒ וְהִשִּׂיג֛וֹ כִּֽי־יִרְבֶּ֥ה הַדֶּ֖רֶךְ וְהִכָּ֣הוּ נָ֑פֶשׁ וְלוֹ֙ אֵ֣ין מִשְׁפַּט־מָ֔וֶת כִּ֠י לֹ֣א שֹׂנֵ֥א ה֛וּא ל֖וֹ מִתְּמ֥וֹל שִׁלְשֽׁוֹם׃
By nie ścigał mściciel krwi zabójcę, gdy rozgorzało serce jego, a nie doścignął go na dalekiej drodze, i nie zabił go, - gdy przecież nie zasłużył na śmierć, nie będąc nieprzyjacielem jego przedtém;
Rashi
פן ירדף גאל הדם LEST THE AVENGER OF THE BLOOD PURSUE [THE SLAYER] — therefore I tell you to prepare the way for you and [to prepare] many cities of refuge.
Ibn Ezra
strike him mortally [literally: strike his life] an elliptical construction, meaning “strike him a mortal blow ” — i.e., strike him a blow in a place from which his life will leave him.
Rashbam
פן ירדוף, these words refer back to the need to establish three such cities as outlined in verse 2. If there were to be only one such city of refuge, the avenger in his pursuit would be more likely to succeed.