Sometimes Mercy is Justice, if the person feels contrition and guilty. The forgiving mercy extended to them will haunt then and touch them. Hopefully change them for the better.
But I believe in Justice more. But more like to be "just", with the proper review of the offense and proper trial. Islam makes it hard to try an offender injustice as it requires great evidence to put a person through the justice system.
In legal terms-- I prefer Mercy, as nothing like forgiving that touches the heart and changes a soul. Alaaah swt is most forgiving, therefore, in our creators likeness, we must be merciful as well.
However, in terms on dealing with life and people, I think being just to people is very fine. I think I am like that. Only a realist, with morals, who see clearly, without self delusion can be just. Now, u cannot be just if u r not merciful. I think they both go hand in hand. You do have to have a heart to be a good justice.
