Shooting from Mercury to Venus: On Dante's Paradiso

How can just vengeance itself receive a just punishment? This is the major question in the seventh canto of Dante’s Paradiso. In this second sphere of heaven, named Mercury, those who sought worldly fame and the active life at the expense of the contemplative life share their mercantile understanding of the relationship of just actions.
An eye for an eye; currency in exchange for an object; or two objects of equal value being exchanged--this is the image we receive: a necessity for balance and fairness is what is conveyed by the participants in this sphere. This is perhaps a continuation of Dante’s earlier discussion over the conditions of altering one’s oath. The theme in this sphere is therefore the proper measure or balance between two parties, which the practically minded occupants of this sphere would know best. Since this is still one of the first three spheres marred ever so slightly by sin, those here upset the natural balance and therefore suffered slightly for it. In the same way, those who take just vengeance would do well to understand that in taking vengeance they allow another party (the one avenged upon) the very same claim (the right to take just vengeance themselves)! This notion, however, is expressed in a curious way--because it turns out that the underlying question for Dante is, "why did God become man?" And "why is it that in killing God, man was forgiven rather than eternally damned?" 

Dante first describes Jesus as God descended to earth as a man. He then claims that God as a man must be the most just creature on earth (Par 7.40-42), and therefore that to murder him would be the most unjust or unfair act possible on balance. 

Thus, if the penalty the Cross inflicted
is measured by the nature He assumed,
no one has ever been so justly stung;
yet none was ever done so great a wrong,
if we regard the Person made to suffer,
He who had gathered in Himself that nature.
(Par 7.40-42, Mandelbaum tr.)

How, then, does his death remove sin from man rather than forever mar his soul? Because that is what the scholastic reasoning would suggest: Jesus is the most just man on earth. Justice involves giving to each his due. Therefore, Jesus deserved the greatest thing which can be given. And he was murdered. Now, either murdering someone is the greatest gift one can give (it isn't) or we need a better answer to this question. Beatrice has just such an answer for us.
Since God is defined by Dante as the greatest being in creation, He therefore does not need or desire any charity or gift from man. In fact, to give him anything other than to have unjustly murdered Him, claims Beatrice, would have been to limit the grace of his gift in return to man! Because as God is the greatest being, and giving the greatest charity would befit the greatest being rather than receiving it, and as charity is the greatest virtue, God therefore gives the greatest charity there is to the least deserving possible creature. And in order for God to give such charity, man must commit the ultimate sin or choose to kill God in order that God may forgive man for turning his back on God. And this makes sense in a convoluted way, because if God repaid an injury for an injury, God would then have been injured by man, but as God has a perfect and immortal nature in the scholastic tradition, He cannot, by definition, be affected or changed or wronged, technically speaking. God, therefore, could not suffer an injury from man, but as the ultimate force in the universe, he could forgive man for what appeared to be the ultimate crime, because since there was no injury, there was no crime. 

Man, in his limits, could not recompense;
for no obedience, no humility,
he offered later could have been so deep
that it could match the heights he meant to reach
through disobedience; man lacked the power
to offer satisfaction by himself.
Thus there was need for God, through His own ways,
to bring man back to life intact—I mean
by one way or by both. But since a deed
pleases its doer more, the more it shows
the goodness of the heart from which it springs,
the Godly Goodness that imprints the world
was happy to proceed through both Its ways
to raise you up again. Nor has there been,
nor will there be, between the final night
and the first day, a chain of actions so
lofty and so magnificent as He
enacted when He followed His two ways;
for God showed greater generosity
in giving His own self that man might be
able to rise, than if He simply pardoned;
for every other means fell short of justice,
except the way whereby the Son of God
humbled Himself when He became incarnate.
 (Par. 7.97-120. Mandelbaum tr.)

Dante then goes on to consider why it is that other substances like fire, earth, water, and air and those things that are made of them become corrupt and do not remain eternal like angels and celestial substances. Beatrice says because they are created and that which has come to be must pass away. That which begins must end. The only reason a human is somehow exempt from this is because the Divine breathes his own essence into the human, so when the being (form/soul and matter/body) ceases to be, the eternal part of the human, its nature or form, which came from God, will simply continue to be, but in a much different way as it will have no body. Dante then says that this explains well how one will be resurrected in a similar fashion to how Adam and Eve were created--by having spirit breathed into matter, or by God himself, however He does it.