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.
It's never impossible, but in e.g. a hostage situation, this may result in a lot more innocent deaths.
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?
