Because I have also wondered and never understood how God would NEED to have Jesus sacrificed when human sacrifice itself is a bad thing.
That is a bit tougher one to answer.
First we need to ask why human sacrifice was a bad thing.
Usually the context of human sacrifice was offering one's children to other gods, such as Molech.
The idea was to essentially "bribe" the god with something dear to you to appease them, in exchange for blessings, good crops, out of fear, etc.
The gods were those who must be appeased and man were there to serve them.
In the fulfillment God set's this on its head. We have nothing to offer God, nothing to bribe Him with, we have no righteousness of our own and nothing to bring.
He Himself offers the gift of His Son for us, the opposite of the demanding gods.
Rom 3:23 for all have sinned and fall short of the glory of God,
Rom 3:24 and are justified by his grace as a gift, through the redemption that is in Christ Jesus,
Rom 3:25 whom God put forward as a propitiation by his blood, to be received by faith. This was to show God's righteousness, because in his divine forbearance he had passed over former sins.
He does so out of love, the opposite of the gods:
Joh 3:16 "For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life.
So apart from the fact that sacrificing people is horrible in itself, it also would give the exact opposite of the lesson God wanted to convey.
The sacrifices were substitutes for the sinner. The sinner would confess his sin and another would pay the price. If they gave their children they might view it as bribing God's favor. Instead it was grace in that this other life was allowed to atone for their sin.
Now some don't hold the the "substitution" model of atonement. But it certainly seems that the blood of Jesus, His sacrifice for us, is what cleanses us. Jesus died for our sins. He bore our sins in His body. He became sin for us so that we might become the righteousness of God.
2Co 5:21 For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God.
Isa 53:4 Surely he has borne our griefs and carried our sorrows; yet we esteemed him stricken, smitten by God, and afflicted.
Isa 53:5 But he was wounded for our transgressions; he was crushed for our iniquities; upon him was the chastisement that brought us peace, and with his stripes we are healed.
Isa 53:6 All we like sheep have gone astray; we have turned--every one--to his own way; and the LORD has laid on him the iniquity of us all.
We are healed by His wounds.
1Pe 3:18 For Christ also suffered once for sins, the righteous for the unrighteous, that he might bring us to God, being put to death in the flesh but made alive in the spirit
Just as the innocent lamb died in the place of the sinner in the type so Jesus died once for sins, the Righteous for the unrighteous. This was the means of reconciling us to God.
and no reference point from which God could judge Lucifer or those who chose the prideful way. Including the judgement placed on the discisions of angels.
