This is at best an argument against having poorly-thought-out laws like "Anyone who intentionally kills another loses his head", not one against the death penalty in general.
I say this not because I'm pro-death-penalty; I say it because it's a terrible anti-death-penalty argument. There's much better arguments than such word play so simple a child can see through it.
(Cards on the table: My personal position is that, actually, I'm OK with the death penalty in situations where a society can not afford to keep the murderer alive, or the society does not have a reasonably expectation of being able to keep the murderer from murdering again. But I do not merely mean "the society would be inconvenienced", I mean, the normal state through history is that the society literally can not afford to feed a murderer or have a realistic expectation of holding them; a tribe 5,000 years ago of 100 people literally can not afford it. However, in the 21st century, every major civilized society in which we might even be having this debate is very far away from that problem, having both abundant resources to feed and house them, and very realistic expectations that they can indeed keep them away from innocents. Prison breaks today are big news, and still don't happen all that often.)
For starters, if we accept state sanctioned killings, then any argument that a life is inviolable goes out the window. With capital punishment, the state is saying that taking lives can be acceptable.
> Humanity has been distinguishing between "murder" and "killing" for millennia
Yes, and a major distinguishing factor has tended to be premeditation and intent. An execution is of a nature that would fit in the worst category of murder if it wasn't for the fact that it was ordered by the state.
An execution has all the hallmarks of something that would tick every possible mark of a particularly heinous murder otherwise. Yet even when we know that innocent people get executed too, it is tolerated when carried out by a state.
Very few people argue that. Even fewer people who are pro-capital-punishment would argue that, for obvious reasons. An argument that only convinces those who already agree isn't even an argument.
"An execution has all the hallmarks of something that would tick every possible mark of a particularly heinous murder otherwise."
No, it doesn't. You're just doubling down on the sophism. Again, let me emphasize that if anything, I'm angry that this is a bad argument against capital punishment, not as a defense of capital punishment. Everyone from philosophers to the man on the street have understood the differences between cold-blooded murder, murder in the heat of passion, execution, warfare, and self-defense for millennia. Nobody who is capable of making those distinctions is going to be convinced to see all killing as equal, when even the man on the street considers it a rather unsophisticated position.
See also why the animal rights argument that "if it's wrong to murder people for premeditated reasons it should be wrong to kill animals to eat them" hasn't gotten any significant traction. Appealing to people to deliberately dumb down their morality for that to work isn't a good argument, especially when, again, it's not even that sophisticated a difference we're talking about here. We're not into weird Latin terms, reading 19th century existential philosophy, or even arguing what "begging the question" technically means, we're talking stuff that your average 12 year old could explain.
The response seems to be that that's a bad argument, they don't really believe that there's no distinction, and it would be stupid to believe that.
If I don't see a distinction of moral significance between the death penalty and, say, the CEO of a company deciding to painlessly execute one of their reports that they discover has gotten away with serious embezzling for multiple years, scorn doesn't help me understand the distinction someone else does see.
And it is one I agree with. Personally I consider juries that convict in capital cases, and prosecutors that seek the death penalty, to be no better than murderers.
I'd be fine with that. A state can confer value to some extent and still be illegitimate and immoral.
> To be consistent, you would have to argue that taxation is equivalent to armed robbery,
No, because taxation do not require the use of arrest as an enforcement mechanism. I am perfectly happy to consider the use of prison for failing to pay tax to be abusive.
I would, one the other hand, also consider it perfectly reasonable to place restrictions on most societal benefits from someone failing to abide by tax law, and to resist someones attempts to make use of them. Including denying them access to shared resources such as public roads.
If someone is able and willing to put themselves entirely outside of society, on the other hand, and so don't want to pay taxes as a result, I'd support that.
> law enforcement arresting someone is equivalent to kidnapping, and so on.
I do believe all countries massively overuse arrest and incarceration, yes.
I would tend to find it immoral except where failure to do so puts innocent people at substantially higher risk. Even then I believe we have a strong moral duty to seek the least invasive intervention, which would e.g. imply that if/when we are forced to use prisons they should restrict peoples freedoms as little as possible.
Even more so because the evidence is that strict prison systems produces more reclivism than more lenient system. As such, a lot of our "justice" systems not only harm those imprisoned but also inflicts further harm on society by doing a poor job at reducing reoffending, despite the fact that we know better methods.
We will not avoid inconsistencies, but we can seek to minimise them to only those circumstances where we have no other recourse. We have a recourse other than murder. For starters, we have prison. Most murderers that are released from prison never kill again, so the risk is minimal. Arguably low enough that capital punishment quite possibly kills more innocents than releasing murderers from prison would.