Parashas Masei · Passuk 35:16 · TT note 3
Chabad.org — English translation + Rashi
Torah Temimah points to Sifrei (TT note on “מוֹת יוּמַת הָרֹצֵחַ”).
English: “The murderer shall surely be put to death” — one might think that because it is written “or with an iron implement he struck him” he is liable only for an iron implement; from where [do we know] that if he threw upon him metal pieces or unformed lumps and killed him he is liable? Scripture says “he is a murderer; the murderer shall surely be put to death” — in any case.
Expansion of liability beyond a finished iron tool to any iron mass that kills.
Death caused by any form of iron (finished tool or raw mass) renders the killer a murderer liable to capital punishment.