IF YOU BREAK A GOOD LAW, justice must be invoked not only for goodness' sake but for the good of your own soul. Justice may consist of paying a price for what you've done or simply of the painful knowledge that you deserve to pay a price, which is payment enough. Without one form of justice or the other, the result is ultimately disorder and grief for you and everybody. Thus justice is itself not unmerciful.
Justice also does not preclude mercy. It makes mercy possible. Justice is the pitch of the roof and the structure of the walls. Mercy is the patter of rain on the roof and the life sheltered by the walls. Justice is the grammar of things. Mercy is the poetry of things.
The cross says something like the same thing on a scale so cosmic and full of mystery that it is hard to grasp. As it represents what one way or another human beings are always doing to each other, the death of that innocent man convicts us as a race, and we deserve the grim world that over the centuries we have made for ourselves. As it represents what one way or another we are always doing not so much to God above us somewhere as to God within us and among us everywhere, we deserve the very godlessness we have brought down on our own heads. That is the justice of things.
But the cross also represents the fact that goodness is present even in grimness and God even in godlessness. That is why it has become the symbol not of our darkest hopelessness, but of our brightest hope. That is the mercy of things. Granted who we are, perhaps we could have seen it no other way.