Numbers 31:19

Outside the camp. So that they would not enter the Sanctuary courtyard.

Whoever has killed a person. R’ Meir said, “Scripture refers to a person who kills with an object capable of receiving contamination, and Scripture teaches that a utensil defiles a person when it is in contact with a corpse as if he had touched the corpse itself. Or, perhaps even if he shot an arrow at him and killed him? The Torah, therefore, teaches, “and whoever touched a corpse.” It compares one who kills with one who touches. Just as the one who touches (receives contamination) through contact, so does the one who kills (receives contamination) through contact.”

Purify yourselves. With sprinkling water, as prescribed for others who were defiled by contact with the dead. For, even those who rule that Gentile graves do not convey contamination in a tent, — as it is said, “you, my sheep, my pastoral sheep, you are man”; you are entitled “man,” he admits that gentiles convey contamination through touching and carrying. Since “man” was only said in the context of tent contamination, as it is said, “a person who dies in a tent.”

You and your captives. Not that the gentiles receive contamination and require sprinkling. Rather, just as you are members of the convenant, so with your captives, when they enter the covenant — should they become contaminated, they will require sprinkling.

But as for you, abide without the camp seven days; whoever hath slain a man, or touched the dead, you shall sprinkle on the third; and on the seventh day both you and your captives,

You and your captives. [You should purify yourselves] in honor of the One Who dwells among you.

WHOSOEVER HATH KILLED ANY PERSON, AND WHOSOEVER HATH TOUCHED ANY SLAIN, [PURIFY YOURSELVES ON THE THIRD DAY AND ON THE SEVENTH DAY]. “Rabbi Meir says: Scripture is speaking about someone who killed with an object which is susceptible of impurity, and the verse [here] teaches you that the object [i.e., the weapon] renders the person impure through [indirect] contact with the corpse as if he had touched the corpse itself., Note 40. I might think that even if he shot an arrow at him and killed him [the same law applies]; Scripture therefore says, and whosoever hath killed any person [and whosoever hath touched any slain]. Thus it compares a person who kills to a person who touches. Just as he who touches [is rendered impure] by virtue of that contact [with the corpse], so is the person who kills [rendered impure] by virtue of his contact [with the corpse, effected through the weapon he holds in his hand, which contact is non-existent when he shoots an arrow and kills him].” Thus far is the language of Rashi.
But his words are not clear to me. For if he killed with an object which is susceptible to impurity but can be [made pure] by immersion, then the person touching it does not contract the seven-day period of impurity, nor does he require the sprinkling [of the waters of purification], since the object [with which he killed] becomes “a father of impurity,”, Note 40, for an explanation of this subject. and the person who becomes impure through it, becomes a “first degree of impurity” [who is only required to immerse himself in a ritual pool and he becomes pure the evening of that same day; and since the verse here requires a seven-day period of impurity, it cannot be referring to such a case]. And if [he killed him] with a metal instrument, we have already been taught that a sword [or any metal object with which a person is killed] is exactly the same degree of impurity as the dead person himself,, Note 40. and therefore conveys a seven-days’ impurity [to anyone who touches it, so there is no need for our verse to repeat this principle]. And if the Rabbi [Rashi] is of the opinion that since he [the killer] touched the instrument whilst it was itself still in contact with the corpse [then even if it was not a metal object] it renders him impure for a seven-day period just as [if he had touched] the corpse itself — such [conveyance of a stricter degree of] impurity through [indirect] contact is only a …