People have different morals, but justice is not so much an abstract idea but something that we can vote and decide on. If there were exceptions to every law, than how could we maintain a society that is truly fair? Plus, justice should reflect what the majority of the population believes to be morally correct. If it does not, the true question is is it really justice? Yes, in a society justice is one of, if not the, most important component.
If there's no justice, then how are we suppose to maintain the society that we humans had built for millenniums? If we do justice, we're not only showing examples to other, but we're also helping to make a better society and better future.Would you sacrifice something major for minor things?
First, there's no real standard for what any of that means. Second, there's no inherent disconnect between the ideas even if vague and open to interpretation. "Mercy" for instance is usually melding of love and law, "Justice" is often a melding of morality and righteousness, and so forth and so on.