ludwig - we accept one and not the other because these are the stuations and circumstances we have to deal with.
In the proverbial ideal world, we would not be having this exchange, because no-one would murder anyone, ever.
However, we are a collection of humans with attendent foibles, opinions, tolerances, dark sides, and so on and so forth, and this is what gives rise to the issues we are debating.
No killing is ever desireable, or even has degrees of desireability in my view, but we have to deal with what we have, and in the case of the officers dealing with gun incidents, they have to deal with things first, and have chance to think and reason about them afterwards.
So although i would love our debate to be a matter of ethics set in utopia, it is not, it is based on our individual viewpoints, which we know differ substantially from others even on here, never mind the wider world.
We all reach accomodations we can live with - some believe (falsely in my view) that they could cheefully dismember the next child rapist with a chainsaw, and sleep soundly every night afterwards.
Others - and I am one - believe that the taking of a life as a punishment holds no merit.
But we all rub along with the systems in place - and at the moment, and for the forseeable future, society agrees with me.