The dispute I am addressing is the objective issue of whether “objectively wrong” has a meaningful definition.
You’re implicitly defining “arguing” to mean “arguing over an objective issue,” which is assuming what you’re trying to prove (i.e., that “objective rightness and wrongness are coherent concepts”). People can heatedly and actively disagree over the rightness or wrongness of something based on their subjective opinions, whether or not you want to call that “argument.”
I don’t mind if we limit the scope of the words “argument” and “dispute” to objective issues, but then the subjectivist would simply say that under such definitions it is impossible to argue over whether an action is right or wrong, because they are not objective concepts. (Which puts us back where we started.)
And the shooter would care if he heard an objection on objective grounds? The most effective strategy would likely be to appeal to the shooter’s self-interest, and avoiding performative contradictions is probably not very high on his list of priorities.
First of all, emotivism is a meta-ethical theory purporting to interpret what people mean when they use ambiguous moral language, so it has nothing to say about what is right or wrong - only about what people mean when they say such words. And the subjectivist’s position is that right and wrong are matters of his own opinion/feeling/etc., so subjectivism would only result in “might makes right” (for him) if that happened to be his view.
If you mean that we shouldn’t want Statist politicians to be subjectivist and instead it would be better if they all followed the NAP, I fully agree. However, when there is no monopoly on force, what exactly is the worry? Under the AnCap vision, the law would be what consumers want and pay for, and we can only make educated guesses that there will be a strong tendency toward natural law. I am fine with such educated guesses, and I find such a scenario rather plausible. However, even assuming “objective ethics” can be coherently defined - which I am doubting more and more - I don’t see what purpose it would serve without the State monopoly.
It strikes me as a relic of the paradigm shift from mini-archism to (p)anarchism, especially when we recall that just a few decades ago minarchism was the dominant libertarian view. Under minarchism, it indeed makes sense to not allow subjective interpretation of the Constitution or consequentialist judgments on whether the president needs “emergency powers,” but in the new paradigm you’re no longer forced to live under one master, and moreover there is no monopoly capable of ensuring universal compliance with any “objective” standard.
In other words, subjectivism may have been a proximate cause (of government power abuse), but the core problem was the monopoly on force.
This only works for proving propositions that have coherent meaning, but the coherency of the meaning is the very issue.