Well, I think the above serves to prove my point. You say that murder, or the unjustified taking of another life is wrong by definition. That is what I would refer to as a moral value or a moral judgment. In saying that murder is wrong by definition, you are making a judgment and are therefore the one judging or making the consideration that murder is not right but that it is wrong.
No, I´m just pointing out the connotations of a word. "Murder" is a term for "wrongful killing" - so "murder is wrong" is just a tautology. I´m just the messenger here.
Now, people consider certain behaviours right or wrong, and different people consider different behaviours right or wrong. I´ll give you that.
In the same way, people consider certain situations of killing wrong and therefore call it "murder" (wrongful killing), and different people consider different situations of killing "murder" (wrongful killing).
I believe the only question now is: Is murder objectively wrong, or subjectively wrong?
You are still employing a tautology. That´s not the best choice of an example and complicates the discussion.
What I know is: People consider certain forms of killing "murder" (wrongful killing). IOW they consider it wrong. They make their subjective moral judgements. Thus, the case for there being subjective moralities has been made.
Now, go ahead and make your case for the existence of "objective morality" (not without giving a positive definition beforehand, please).
We all make moral judgments, we all act as judges.
Yes, subjective morality is observable and observed.
The question is,
(1)are we all judges equally, in the sense that no person's judgment supervenes over another's, or
(2) is one human being's judgement supervenient over all the rest of humanity,
(3) or is a collective majority of human being's judgement supervenient over humanity,
(4) or is there one outside of and beyond humanity whose judgement's are supervenient over humanity?
ad (4). I don´t know that there is a judge outside and beyond of humanity. If you can demonstrate that there is such a judge, we can start looking at the implications. (In any case, that would be reversing your previous approaches: So far your premise was "objective morality exists (therefore a super-judge must exist)", now your premise would be "a super-judge exists (therefore objective morality exists). It all appears to be very circular, I must say.
Even if there is such a judge, the term "objective" doesn´t apply - it´s still this judge´s subjective morality. We may or may not find it appropriate to submit to this person´s judgement - but that´s a different question altogether.
ad (2). I don´t know what could possibly be the criteria for one person´s judgement superceding that of another.
ad (3). How people deal with the fact that their values differ (e.g. if they decide to let a monarch decide, or if they choose one of the available concepts of letting the majority decide the rules) is a
pragmatic issue of handling the fact that people disagree in their subjective values.
(ad 4), however doesn´t preclude any of the other options. Even with there being and outside and beyond judge, the fact that our personal judgements are subjective persists, and it can´t be concluded that any human´s judgement supercedes that of another.
Anyway.
As requested in the OP (and as you have conceded) the case for the existence of moral subjectivity and the fact that morals differ depending on time and place (relativity of morals) has been made.
For the time being (i.e. in the absence of a case being made for there also being an "objective morality") I am merely acknowledging these facts.
Now, will you start making your case for "objective morality" existing along with subjective moralities any time soon, or will you keep beating around the bush?
I´m eagerly awaiting your arguments.
(If an "objective morality" - and what its values are, in particular - could be shown to exist, this surely would come in handy. Thus, in my own best interest, I am wishing you the best of luck with that.)