Tension between Divine Justice and Divine Mercy

Why do divine justice and divine mercy appear to be in tension with each other, and how does Anselm resolve this tension?

Presumably, a just God must punish the wicked consistently. However, out of His mercy, He spares some of them from the penalty. Hence, this seems to compromise His just nature. In Ch IX of The Proslogium, Anselm begins to delineate this tension and suggests a resolution by subsuming mercy under justice. He first conjectures that goodness and justice in God are coterminous: "For, if thou art compassionate only because thou art supremely good, and supremely good only because thou art supremely just...". And, for God's superlative goodness, he spares both the good and the bad rather than just the good. Anselm maintains, "...it could not be that thou shouldst be good...not by sparing...the wicked also...it is just that thou shouldst spare the wicked, and make good souls of evil". This also implies by privileging clemency to the wicked not arbitrarily, but as an opportunity to improve, God is merciful in a just way and withholds goodness. Lastly, in Ch XI, Anselm is unsure of how different groups of equally proportionate sinners sometimes face varying deserts.