Argument from evil, restated

[Neo-Scholastic]

...trying to mix god's mercy and grace into his justice. The resulting system does of course seem ludicrous, as justice is deserved, while mercy and grace are undeserved.

As I understand Justice and Mercy are intertwined and distinct from Grace. Justice is to punish the wrong-doer in accordance with law. Mercy is taking into mitigating circumstances. For example, suppose the law says the punishment for murder is death. But the judge recognizes that, while the defendant is guilty, he is also mentally challenged. Thus the wise judge tempers the sentence and shows mercy on the the defendant.