I think that by and large we see justice and mercy as being two different things, even at times opposite things. Not without reason, either. Justice seems to be seen more as reaping the consequences for one's actions, particularly in the sense that one is punished for breaking laws. Mercy is seen as either not being punished, or in someone else choosing to suffer the punishment instead of the one who broke the law.
A simple example may be best--a child plays with something he has been told to not play with, and breaks it. Justice would be punishing the child--no playtime, spanking, sitting in a corner, whatever may be deemed appropriate. Mercy would be to not punish the child, or to at least not be severe about it.
This isn't a nonsenical view of the two things, but I also wonder, are their not times when justice and mercy are, in fact must be, the same things?
Consider the child again. If the parents choose to punish him, are they being just or being merciful? If they had previously said "Do not play with that thing, or you will not have playtime", then if they punish him in such a way for disobedience, then that is just.
I would contend as well that it is mercy, too, or at least could be. If the parents think "We do not want our child to be disobedient and rebellious, as that will likely cause him to be a bad person", then the idea of enforcing a punishment because not only an act of justice, but also of mercy--they want their child to respect property, to obey his parents, to learn lessons he needs to learn. In fact, if they did not punish him, but instead allowed him to behave in such a way, it is likely (not a certainty, but likely) that he will behave in worse ways as he grows older, and if he learns the lessons, they will be by more severe ways.
I want to take it a bit further, and I fear this will be hard, if not harsh. I would contend that for some people the only mercy that can be shown to them is justice. They have simply reached the point where mercy as commonly seen is fruitless, and they have even taken advantage of such acts of mercy in the past to continue on their own roads to doing wrong. They have been kept from suffering consequences of their wrong choices, or at least the full consequences, and so have not learned the lessons, but have continued on doing with is wrong.
A point was reached in the time of the prophet Jeremiah when God told the prophet to not pray for the people. God still loved His people, I do believe, but they had come to where judgment, justice, could not be averted.
I don't like to think that. It is open to so much misunderstanding, even abuse, but I think it is a place can be reached by some people, where they must reach the very bottom before they can really be helped.