I was thinking while I was away (since post #171) that God could have crated a perfect man with no prior life in heaven, but arranging for the sacrifice of that man would have been merciless to the one sacrificed. But by transferring a spirit son to human life, he could redeem the sacrificial Lamb as well. Because the person sacrificed would still have the right to life, just not the right to human life.
Instead of using just any angelic son, he used his best, his first and only direct creation. Showing that there is nothing he will hold back from us if it can be satisfied within his senses of love, justice and wisdom.