I believe to be the "Second Adam". Christ had to become one of us (to be a "son of man") and suffer under the bondage of the law of sin and death (to become a curse for us) to be the Firstborn of many brethren.I guess the question is why Did Christ have to die? He had to die. If not for punishment if our sin, why?
God could not justify man because men sinned. But God did justify Christ in His obedience even to death. Christ's birth (incarnation), life, temptation, suffering, death and resurrection are absolutely necessary. But not as an object of divine punishment.
The short answer is Christ had to suffer under the law of sin and death to free us from its bondage.
Why would Christ have to suffer and die as divine punishment for God to forgive our sins?