what are we supposed to say again when christians ask us where we get our morality?

[GrandizerII]

And do you know what God's will is exactly?

We know His decreed will because He explicitly decreed it; hence the name.

The problem is you don't. Therefore, an inaccessible/unknowable source of objective morality is pretty much an equivalent of a nonexistent source of objective morality. You have no objective morality there.

I need to know what verse(s) you are referring to. All killings that violate God’s decreed will are objectively wrong.

And all killings that supposedly are according to his will are objectively right, right?

Is this objectively right then?

1 Samuel 15
1 Samuel said to Saul, “I am the one the Lord sent to anoint you king over his people Israel; so listen now to the message from the Lord. 2 This is what the Lord Almighty says: ‘I will punish the Amalekites for what they did to Israel when they waylaid them as they came up from Egypt. 3 Now go, attack the Amalekites and totally destroy[a] all that belongs to them. Do not spare them; put to death men and women, children and infants, cattle and sheep, camels and donkeys.’”

And his opinions regarding what the Bible says.

That’s a rather meaningless objection, that’s like trying to say that 4 is not objectively an even integer because some students are of the opinion that odd numbers are exactly divisible by 2.

No, not the same. Don't be silly. 4 being an integer is not based on one's subjective opinion. It is an objective fact.