The Apostle John's words, God is love, have been so oft repeated that they can seem trite and shorn of real meaning, as though the Beloved Disciple were some sort of aging hippie, hanging out on Patmos and flashing the peace sign. However, if we take his words as a profound statement of Truth, then all of His other attributes must be manifestations of Love.
If you then work through the implications of this, as the Scotsman did, other truths emerge. God's justice is not opposed to his mercy; they are two sides of the same coin. If, being just, he punishes, that punishment itself is merciful, for, rather than being merely retributive, it is exactly what the sinner needs.
Our job is to imitate Jesus, to be perfect, even as our Father in Heaven. Therefore, we cannot be just unless our justice, too, is a manifestation of love. Does this mean our justice will be weak, enabling sinners to go on sinning? Not at all, for that would be unjust to them!
If "justice" can be defined as "the setting of things right," then the ultimate outcome of justice is a sinner's repentance--not merely a "feeling sorry," but a true turning away from sin. If that is accomplished without punishing the sinner, is the justice somehow incomplete? Does justice demand some retributive element, without which it is not just? Often enough, the victims cry out for retribution; but perhaps it is for just this reason that God said vengeance is His; perhaps His vengeance is higher and better, more perfect than ours.
I respect both sides of the argument; however, consider the following. If you're like me, there are sins in your past which, even to think of, cause you stabs of pain. Now, imagine the worst man of whom you've ever heard, and imagine that he finds God--or God finds him!--and he repents. The monstrousness of his past sins is now clear to him. Can you imagine a worse punishment than that?
I'm reminded of the true story of a neo-Nazi who lived in the Western U.S. twenty or thirty years ago, and for years spewed out virulent, violent, hateful, anti-Semitic tracts. Eventually, he grew old, penniless, and blind, and a Jewish couple who heard of him took him into their home and showed him love. He lived out his years with them, and, repenting of his past life, loved them in return.
Thus was Justice done.