John Gill Commentary Numbers 35:16

"But if he smote him with an instrument of iron, so that he died, he is a murderer: the murderer shall surely be put to death." — Numbers 35:16 (ASV)

And if he smite him with an instrument of iron, so that he die As with an hatchet, hammer, sword, knife, &c.

he is a murderer ; the instrument used by him, and with which he smote, shows that he had a bad design, and intended to kill, or he would never have smitten a man with such an instrument:

the murderer shall surely be put to death ; be condemned to death, and be executed, by the order of the civil magistrate, according to the law in (Genesis 9:6) and not be allowed the benefit of a city of refuge.