[a little frown, but he says] They're very powerful, and they can't be killed like humans can. If you kill the body, you kill the human host, not the evil inside. Then the Noah will simply possess someone else. All human bodies have the Noah genes dormant inside them, so it could be... anyone.
Right now, we can't eliminate their threat, only create more victims. So -- we've been watching them, instead, making sure they don't hurt anyone here. I'll know if they create any akuma, at least. [firmly]