I'm sorry, but I believe everyone is missing the point.
Why did God, God the Son, "fully God", etc. need to die for our sins?
The man Jesus of Nazareth was born having the Holy Spirit as his Father (Luke 1:26-35).
I don't see this as meaning that this man was "fully God".
“Adam, who is a type of Him (Jesus) who was to come†(Romans 5:14).
Pre-fall Adam ... and ... Jesus of Nazareth ... were BOTH sinless men.
God's PLAN # 1 (full of compassion and grace) ...
Produce a perfect sinless human who would stand in as our Substitute,
and be the perfect Sacrificial Lamb, and die for the sins of the human race!
I'm assuming you understand why man could NOT accomplish this for himself.
I believe continuing God's ancient blood covenant also played a part in this.
God's PLAN #2 ...
Have God Himself become a man, a "fully God" man.
WHY did God choose to do the second plan? ... Was it necessary?
God produced His sinless man, which was sufficient to provide for man’s salvation.
Originally, before his fall, the man Adam was without any sin.
“Adam, who is a type of Him (Jesus) who was to come†(Romans 5:14).
Adam’s fall resulted in sin, and death, and the condemnation of all men.
Jesus Christ’s righteous act was a free gift, which resulted in the justification of life for many.
Romans 5:
12 Therefore, just as through one man sin entered the world, and death through sin,
and thus death spread to all men, because all sinned —
13 (For until the law sin was in the world, but sin is not imputed when there is no law.
14 Nevertheless death reigned from Adam to Moses, even over those who had not sinned
according to the likeness of the transgression of Adam, who is a type of Him who was to come.
15 But the free gift is not like the offense. For if by the one man’s offense many died,
much more the grace of God and the gift by the grace of the one Man, Jesus Christ,
abounded to many.
16 And the gift is not like that which came through the one who sinned.
For the judgment which came from one offense resulted in condemnation,
but the free gift which came from many offenses resulted in justification.
17 For if by the one man’s offense death reigned through the one, much more those who receive
abundance of grace and of the gift of righteousness will reign in life through the One, Jesus Christ.)
18 Therefore, as through one man’s offense judgment came to all men, resulting in condemnation,
even so through one Man’s righteous act the free gift came to all men, resulting in justification of life.
19 For as by one man’s disobedience many were made sinners,
so also by one Man’s obedience many will be made righteous.
20 Moreover the law entered that the offense might abound.
But where sin abounded, grace abounded much more,
21 so that as sin reigned in death, even so grace might reign
through righteousness to eternal life through Jesus Christ our Lord.