Christ the Antidote
I have a longer post in preparation with some thoughts on the theory of retributive punishment, but I wanted to share what I think is a helpful word picture (pun intended!) derived from St. Athanasius' classic On the Incarnation of the Word that can help understand why it was necessary for Jesus to suffer and die, without that suffering and death being penal, i.e. a punishment inflicted upon him.
According to Athanasius, the ultimate goal of the incarnation of the Word was to purge human nature of the corruption of sin and revive it. But human nature could not be cleansed of corruption from the outside: renewal had to take place from within. The only being with the kind of life which could renew the life of the human creature was the Word. Thus the Word took on a body and allowed that body to suffer all the consequences of sin, including death, so that the Life of the Word could then neutralize and destroy them entirely.
A helpful way to think about Athanasius' argument is in disease terms. Imagine a healthy person deliberately exposing himself to a deadly disease, knowing that he has the vitality and strength to neutralize it and thus produce an antidote. To do so the person has to actually suffer from the disease. It is painful for the body to go through the systems as the immune system furiously develops the antidote (it goes without saying this analogy should not be pressed too far!), but ultimately the disease is completely neutralized, the person springs back to health and can now 'infect' others with the antidote.
Jesus suffered death so that he could destroy it and infect us with life. He is truly the Great Physician.