Abraham was raised in a culture where it was accepted that God(s) accepted human sacrifice. This was common in the area Abraham was raised and since it was an oral, not written tradition it’s conceivable, even likely that when Abraham understood God “telling him” to sacrifice Isaac it was the same as if I were to tell you that God “told me” not to lie.

God has never parted the sky and appeared to me telling me verbally not to lie, if I say “God told me not to lie” or “not to steal” or whatever else I say “God told me” to do or not to do, I’m referring to religious tradition and not literal direct communication.

So according to the tradition he was familiar with Abraham makes the journey up the mountain and just as he is about to murder his son… God, merciful God, appears and tells him to stop. God gets fed up with thousands of years of humans murdering their children for his sake and finally does something about it by telling Abraham emphatically STOP!!!

Even more likely, there was no angel, and Abraham just realized at the top of that mountain just how sick this business of human sacrifice was.

As he looked into the eyes of his child with love, he realized how necessary it was that his people stop sacrificing their children to God.

Knowing that the only way the Hebrews would quit practicing this horrible behavior was if God told them to stop (and it had to be a supernatural direct communication with God) he made this up about the angel.

Thank God (or Abraham) that he did.