Midras Ulangan 19:11: Mekhilta DeRabbi Yishmael, Sifrei Bamidbar, Sifrei Devarim & Yalkut Shimoni

And whence is it derived that it must be in a locus which is critical to life? From (Devarim 19:11) "and he lie in wait for him and he rise up against him and he smite him mortally," whereby we are apprised that it (the weapon) must be of killing potential in a locus which is critical to life.

R. Yitzchak says: Even a man who intends to smite one and smites another is not liable — until he makes it clear that it is this man that he wishes to smite, as it is written (Devarim 19:11) "and he rise up against him and he smite him, etc." What, then, is the intent of "and if men fight, etc."? From (Leviticus 24:17) "And if a man smite any soul of man" I would understand even an eight-month birth, (who is destined to die); it is, therefore, written ("and if men fight," to indicate that) he is not liable unless he kills one who is (otherwise) destined to live.

(Bamidbar 35:16) "And if with an iron implement he kill him (intentionally) and he die, he is a murderer.": What is the intent of this? From (17) "And if with a hand-stone … (18) Or if with a wooden implement, etc.", I might think that he is liable only if he killed him with one of these. Whence would I derive (the same for) iron? It is, therefore, written "And if with an iron implement he kill him, he is a murderer." — But (even) without this I can derive it a fortiori, viz.: If he is liable for killing him with stone or wood, how much more so with iron! — But if so, I would say: Just as a stone must fill the hand (thus "hand-stone"), so, iron. It is, therefore, written with an implement of iron, (of any size). It is revealed before the Holy One Blessed be He that iron of any size can kill, wherefore "hand" is not written (in that connection) — even a needle or a pin sufficing. This tells me only of his killing him with iron. Whence do I derive the same for his throwing at him metal balls or lumps? From (16) "He is a murderer; die shall die the murderer" — in any event. (17) "And if with a hand-stone, whereby he can die, he strike him and he die, he is a murderer. Die shall die the murderer.": What is the intent of this? From (Shemot 21:18) "And if men quarrel and a man strike his neighbor, etc." I might think (that this obtains) whether he strikes him with something which is or which is not lethal. It is, therefore, written "And if with a hand-stone (i.e., one which fills the hand), whereby he can die, he strike him." Scripture hereby apprises us that he is not liable unless he strikes him with something which is potentially lethal. I might think, even (if he strike him) on a (body) site (a blow to) which is not mortal. It is, therefore, written (Devarim 19:11) "And if a man hate his neighbor … and he strike him mortally" — whereby we are apprised that he is not liable unless he strikes him with something which is potentially lethal and on a (body) site, injury to which may result in his death. This ("And if with a hand-stone") tells me that he is liable only if he kills him with a stone. Whence do I derive (the same for) his rolling rocks or pillars over him? From (17) "he is a murderer — die shall die the murderer" — in any event.