Send police to deal with the situation. If that is not possible for some reason, then that is the problem that needs fixing. You don't need murderbots.
I suppose a better question would be, why do you feel that in this particular situation - a murderer on rampage - you still feel that it is "deeply evil" to kill them in such a way that they "have no way to see it coming", to the point where you'd prefer other people to risk lives to do it in some other way?