Justice is the concept of getting what you deserve.
Mercy is not giving someone a punishment they've deserved.
So, to the extend that justice results in punishment, mercy is the opposite.
The Christian God is often described as perfectly just as well as perfectly merciful. How can this be? If the quantities J and M are equal and opposite, then no one will suffer punishment (universalism). If not, then not only is the Christian understand of God wrong, but then we also have no way to know which quantity is bigger – are we due perfect punishment? Are we due excessive reward? Do these quantities scale by person? Who can say. Perhaps this is where the admonishment to never judge your fellow man is rooted, for only God will know the interplay between J and M.
It seems the only logically defensible position is that, while the bible might describe some general moral framework, the J+M equation is highly subjective and means each of us has our own morality.
Leaving the previous concepts aside, Justice seems highly warped in the Christian context. When one considers the dimensions of Hell against being a bumbling, ignorant human - how could one ever merit it? Would it not be the same as a court sentencing a profoundly retarded man to death for jaywalking?