We cannot take mercy for granted, as that is the sin of presumption. And, until one goes through the Dark Night, one vacillates between the sins of presumption and despair. One learns what justice really is and what mercy really is.
Mercy is above justice, not in place of...there seems to be confusion as to God's justice being something which is contrary to mercy. It is not, The two, true Attributes go hand in hand.
Summa, First Part, Question 21. The justice and mercy of God
from Thomas Aquinas, a few thoughts...
Now as works of art are related to art, so are works of justice related to the law with which they accord. Therefore God's justice, which establishes things in the order conformable to the rule of His wisdom, which is thelaw of His justice, is suitably called truth. Thus we also in human affairs speak of the truth of justice.
Justice, as to the law that governs, resides in the reason or intellect; but as to the command whereby our actions are governed according to the law, it resides in the will.
The truth of which the Philosopher is speaking in this passage, is that virtue whereby a man shows himself in word and deed such as he really is. Thus it consists in the conformity of the sign with the thing signified; and not in that of the effect with its cause and rule: as has been said regarding the truth of justice.
Justice and mercy appear in the punishment of the just in this world, since by afflictions lesser faults are cleansed in them, and they are the more raised up from earthly affections to God. As to this Gregory says (Moral. xxvi, 9): "The evils that press on us in this world force us to go to God."