It is immoral because if offends the righteousness of God.
This is an unclear standard. Please either offer a reliable metric for determining why things or offensive in this manner or I will be forced to conclude that you are using a standard which yo uh do not actually understand which is not helpful to the conversation.
I can simply say that anything which offends Betty White is immoral but without some way of determining why something would be offensive to her (Betty White's primary moral axioms) this gives us no actionable data and we are right back to having to rely on our own moral intuition to determine what is and is not offensive to Betty White.
Do you want to live as if there is such a fixed standard
Things are not true or false according to my whims. I have no choice but to believe there is no standard unless some useful standard can be offered. Any discussion of whether this standard is objective of course would be entirely seperate.
The thing is we are moral agents but how did we become such agents?
That we evolved the sensibility is a sufficient explanation and the process of evolution (including behavioral evolution) is observable so that is a more reasonable hypothesis than any hypothesis which includes an undemonstrable explanation even if that explanation would be sufficient.
Morality is based on His (the Yahweh's) nature.
Great how do we determine his nature? If we examine the source material (the bible) the Yahweh appears to be a cruel, capricious, jealous, vengeful, genocidal, egomaniacal maniac whose ten most important rules deal mostly with his own vanity and do not address rape or owning people as property at all and elsewhere in the book deals with these issues very unsatisfactorilly.
He commands that we do not kill
Except when he commands that we do.
The 613 Mosaic laws feed off the Ten Commandments and give us feedback as how the commanments work in specific situations that applied to the ANE culture.
You mean like the ones in leviticus detailing the way in which one goes about owning a human being as property in perpetuity and can then pass them down to one's children as inheritance? I feel like the system you are using fails the livability test. I do not want to be next under this system and I don't think you would either if you gave it a little honest reflection but hey to each his own and if you are willing to be owned by master and obey him even if he is cruel and to give him the right to beat you as long as you don't die within a few days then I suppose to each their own.
There is a difference between free will and no will. You still have a will to choose.
How do you justify hairsplitting between these two concepts? It seems like you want to have your cale and eat it to (freedom to choose but no freewill).