The issue with Abe being "tested" is an interestingone because, as we are told, God tests no one.
So the test wasn't for God ( God knew what was gonna happen, hince the angel), it seems that it was more for Abe's benefit, maybe to prove to himself that No command from God is too great.
That said, it doesn't seem that Abe thought that that His son was going to die.
Genesis 22:
The Offering of Isaac
22 Now it came about after these things, that (A) God tested Abraham, and said to him, “ (B) Abraham!” And he said, “Here I am.” 2 He said, “Take now (C) your son, your only son, whom you love, Isaac, and go to the land of (D) Moriah, and offer him there as a (E) burnt offering on one of the mountains of which I will tell you.” 3 So Abraham rose early in the morning and saddled his donkey, and took two of his young men with him and Isaac his son; and he split wood for the burnt offering, and arose and went to the place of which God had told him. 4 On the third day Abraham raised his eyes and saw the place from a distance. 5 Abraham said to his young men, “Stay here with the donkey, and I and the lad will go over there; and we will worship and return to you.” 6 Abraham took the wood of the burnt offering and (F) laid it on Isaac his son, and he took in his hand the fire and the knife. So the two of them walked on together. 7 Isaac spoke to Abraham his father and said, “My father!” And he said, “Here I am, my son.” And he said, “Behold, the fire and the wood, but where is the (G) lamb for the burnt offering?” 8 Abraham said, “God will provide for Himself the lamb for the burnt offering, my son.” So the two of them walked on together.
9 Then they came to (H) the place of which God had told him; and Abraham built (I) the altar there and arranged the wood, and bound his son Isaac and (J) laid him on the altar, on top of the wood. 10 Abraham stretched out his hand and took the knife to slay his son. 11 But (K) the angel of the Lord called to him from heaven and said, “Abraham, Abraham!” And he said, “Here I am.” 12 He said, “Do not stretch out your hand against the lad, and do nothing to him; for now (L) I know that you fear God, since you have not withheld (M) your son, your only son, from Me.” 13 Then Abraham raised his eyes and looked, and behold, behind him a ram caught in the thicket by his horns; and Abraham went and took the ram and offered him up for a burnt offering in the place of his son. 14 Abraham called the name of that place The Lord Will Provide, as it is said to this day, “In the mount of the Lord (N) it will be provided.”
15 Then the angel of the Lord called to Abraham a second time from heaven, 16 and said, “ (O) By Myself I have sworn, declares the Lord , because you have done this thing and have not withheld your son, your only son, 17 indeed I will greatly bless you, and I will greatly (P) multiply your seed as the stars of the heavens and as (Q) the sand which is on the seashore; and (R) your seed shall possess the gate of their enemies. 18 (S) In your seed all the nations of the earth shall be blessed, because you have (T) obeyed My voice.” 19 (U) So Abraham returned to his young men, and they arose and went together to Beersheba; and Abraham lived at Beersheba.
A important and often overlooked point is here:
4 On the third day Abraham raised his eyes and saw the place from a distance. 5 Abraham said to his young men, “Stay here with the donkey, and I and the lad will go over there; and we will worship and return to you.”
Note that Abe says they wil worship AND return to them.
Seems like Abe felt that, whatever may happen, they woudl return.
IMO, this is quite possibly more a story then fact, a story to show the resolve of Abe, his total trust in God's promise.
BUT even if it was fact, what we have here is a case of Abe knowing that God would be true to His promise and that a great nation would come from Him and Isaac.
It seems to me that God asked Abe to offer the ultimate sacrfifce because he wanted Abe to know that He was ABLE to do all that God commanded and that God would be with him, no matter what that command was.