John Gill Commentary Numbers 35:21

"or in enmity smote him with his hand, so that he died; he that smote him shall surely be put to death; he is a murderer: the avenger of blood shall put the murderer to death, when he meeteth him." — Numbers 35:21 (ASV)

Or in enmity smite him with his hand, that he die
Give him a blow with his fist, on some part of his body where life is most in danger, and which issues in death:

he that smote him shall surely be put to death, for he is a murderer ;
and therefore, according to the original law, ought to die, without reprieve or pardon; and notwithstanding this law made for cities of refuge, which were to be denied him:

the revenger of blood shall slay the murderer when he meeteth him :
that is, when he is condemned, as both the Targums of Onkelos and Jonathan interpret it, after a hearing and trial of his case.