We will be reading a great deal about the transmission of ritual impurity in other tractates. But one fascinating aside mentioned on this page caught my eye. That is the special case of transmission of impurity through a sword:
[And whosoever . . . toucheth] one that is slain by the sword, (Num. 19:16) [which intimates], the sword is as the slain; hence it is a principal defilement
We’ll see this again several times in this tractate (Pesachim 19b and 79a). A corpse is a principal source of defilement (that is ritual impurity). The sword which slays becomes impure though the contact with a corpse – that is, but killing someone. It then creates impurity by those who touch it, just as a corpse does.
It’s a powerful and unique case. Thinking about weapons as objects which, when used, defile.