No.
What is "legal" and what is "moral" are often two different things. Some laws have zero grounding in morality. Some are arguably against the society's prevailing moral beliefs.
You just said..
"Those are not murder by the predominant
moral, religious & legal codes of the day, but rather justifiable homicides."
So morals depend on what the society says is moral. ISIS is a society. It creates laws. It governs. I maintains and builds infrastructure. Therefore, according to what you just said, those beheadings are moral.
Personally, I am Roman Catholic, and part of that is a belief that there is an objective moral position, and that it is of divine origin.
The thing I have to admit is that there are tens of thousands of other belief systems that have the exact same belief, but whose particulars of morality differ from mine.
...and that is before we even address those who don't believe in objective morality.
The major faiths are very similar at the core, but differ around the edges. I understand that.
But to conflate that with a belief that I think Assad is morally justified is incorrect. I don't.
I didn't say you believed it. I said if your statement above is correct then he is morally justified. He has a society that also believes as he does or the gassing would not have happened.
What I am telling you is that your position that laws merely codify morality is demonstrably false. What you call "murder"- the killing of your own children- has only been defined as such by a majority of human cultures for about 1% of human history.
You didn't show that, though. The Romans thought it moral to murder children and then the codified it into law. People don't say, "Hey, let's make a law that says X and then we can think it's moral." Rather, they say, "X is moral, so let's make a law that says so in order for there to be no misunderstanding."
If laws are only a codification of morality, then why did that change occur? And how? If morality is objective, it couldn't.
The change occurred because the perception of what was moral changed and then they codified new laws to represent that. As for how, even if there is an objective morality, people are inherently flawed and won't always see it. So people can view things as moral when they aren't.