I agree Justice is adherence to the Law, and mercy is the opposite of justice, breaking the law because of feelings.
God is described as merciful and just, which seems like a paradox, but if you accept God as supreme, then its less so, and that's an honest theologian who says "I don't know". There's far to many who misquote scriptures to "prove" you have to obey this prelate, or follow this exact ritual to be saved.
I like the theologians who don' try to shackle God in mathematics and rules and iron logic, our human minds are to weak to encapsulate whatever the creator is with our trivial concoctions.
I can't accurately describe all the events of my own life, let alone the universe. The doesn't mean the universe (and my life in it) doesn't exist, its just I am flawed.