Where do you get the idea that someone has to be punished for one's sin even if that person is forgiven?
I am glad you asked. God said that Adam had to die if he ate of the tree of the knowledge of good and evil.
Death means separation. So when Adam ate of the fruit he became separated. Separated from what you might ask? From God, of course because Adam remained on earth for another 900 years before he died physically.
A man's spirit is the life within a man so when the spirit leaves the body then physical death occurs.
When a man is separated from God then that person has died or become separated from God.
Jesus did not die to prevent physical death. I personally believe that man has an appointment with physical death. Even if Adam had not ate of the tree of the knowledge of good and evil he was still subject to physical death as was all of physical creation because of physical creation's transient nature.
Man is different than the rest of the life on earth because the life that man received was eternal life. The life of all other living things is temporary in nature because God's life was not breathed into them. When a man dies, or woman for that matter, there life or spirit lives for eternity. That is the way man was created in the image of God and in His likeness.
God does have form and that form is like the form of God in appearance but not in substance. Man is a created being and God is an eternal being. While man existed before the fall he was held guiltless for all that he did. Man's assignment and realm of authority was to have dominion over the earth and God delighted in man and all that he did and approved of his actions.
There came a time when God told man not to eat of the fruit of the knowledge of good and evil but man chose to defy God and to do as he was instructed not to do. Man became an opposer of God. Instead of destroying man, God choose to separate man from Himself.
Sin is the opposition to God. Adam committed an act that was in opposition to God and in so doing caused his on death. In so doing all his offspring were born into a world in opposition to God. Man's dominion was not taken from him so man used the world as man saw fit and not as God has designed it. So the world lies in opposition to God or in sin. Even those things we call good oppose the will of God because they are the will of man and not the will of God. That state of opposition and all choices and acts of man are called sin. Only those acts that are done in obedience to God are not sin.
In order for man to be separated from sin he must first be come obedient to God. The normal state of man is opposition to God. God has in times past had witnesses to himself that he has chosen and they have born witness to God. Man choose to ignore those witnesses but no man has ever been completely obedient and has shown his human nature just as it is today.
It is not within the power of man to redeem himself. Over the centuries mankind has sought a path back to God and so the birth of religion. Christianity is no different. There are many who call themselves Christian but the faith they practice deny the very God that they purport to serve. That is why one cannot look at a particular sect and say this is a Christian and this one is not. The life of Christ is in the individual and not a particular group or denomination. God knows who His children are and who is not.
All who clame to be sinners ether do not know who they are as a result of there faith or are not chrildren of God. The chrildren of God do not sin. Why, because the seed of the Father (God) remains in them and they cannot sin just as Jesus the Christ was born of God and not of man and could not sin. If Jesus could have sinned according to your faith then two things are true to you even if you wont admit it. First, Jesus was not born of God and second you are subject to the law of God.
We as men die physically because we were created mortal not because we a sinners. Man is already dead spiritually unless he is born again and if he is born again he is as Christ is in this world. So I ask, Who is Christ to you?