cont'd  . . . . only from a certain, finite perspective. So we recognize God as love, as omnipotent, as omniscient, as merciful, and so forth, without being able adequately to imagine that all these attributions form an inseparable unity, because they are identical with each according to their being. That means we have a knowledge of God that overstrains our imagination.
However, it does not follow from the unity of God’s justice and mercy that God's justice is completely dissolved in his mercy. Therefore the restitution that is required by justice does not become superfluous as a result of mercy. That is to say that sin is at the same time an insult to God and a violation of the divine order of things.  Justice requires the restoration of the violated order in the sinner himself as well outside of himself. This restoration is carried out by means of a penalty that does not contradict charity, but springs from charity.  
http://www.catholicworldreport.com/Item/4304/