Komentaryong Sumisipi para sa Mga Bilang 35:17

ואם בכלי ברזל הכהו וימות רוצח הוא, “If he had struck him with an iron implement and he died, he is a murderer.” The Torah legislated that if someone killed with a stone or a wooden implement the court will examine if the force used was sufficient to result in death under normal circumstances, i.e. if the victim were healthy. We know this from verse 17: “If he struck him with a hand-sized stone which is sufficiently large to cause death, or with a wooden implement.” These words mean that the court must weigh the relative strength of the blow according to the physique of who administered it and according to the implement employed to strike the victim with. When the blow was struck with an iron tool or weapon, the Torah does not apply such criteria but assumes that anyone who strikes a blow with such an instrument has murderous intentions. Iron instruments, even if as small as a needle are capable of causing death. (Compare Maimonides Hilchot Rotzeach Nefesh 3,1; 3,2, etc.)
If the killer pushed the victim so that he fell off the roof of a building and died, this is a situation covered by the words in verse 20: “if he pushed him out of hatred or hurled him from an ambush so that he died,” the killer is automatically assumed to have had murderous intent. If less lethal instruments caused the death of the victim the court has to assess if the blows inflicted were prompted by hostility. This is why the Torah wrote: “or he struck him in enmity with his hand.”

בשבט WITH A ROD — Scripture speaks of a rod that is capable of inflicting death. Or perhaps it speaks even of one that is not capable of inflicting death (but some-how or other the servant died beneath his hand)? Scripture however states, (Numbers 35:17) in reference to an Israellite (cf. Numbers 35:15) “And if he smote him with a stone in the hand, wherewith he may die, [and he die, he is a murderer]”. Now is not the following statement a logical conclusion à fortiori? How is it if one has killed an Israelite whose case is more stringent (inasmuch as the leniency mentioned in v. 21 is not applied to it)? He is not subject to the death penalty unless he smote him with an instrument capable of inflicting death and unless it be a limb through the striking of which by such an instrument he is likely to die! Then in the case of a servant where (as can be seen from v. 21) the conditions are less stringent, does it not follow all the more that he is not subject to the death penalty? (cf. Mekhilta).