Many people who reject the command to forgive unconditionally claim it makes a travesty of justice. They are mistaken, and though it's true people sometimes act contrary to justice in the name of forgiveness, that's not a knock against forgiveness, but against mistaken ideas of forgiveness.
Why doesn't forgiveness make a travesty of justice? Forgiveness is the renunciation of a just claim to some good due from another. Forgiveness presupposes justice. If I don't know what is just, I can't know whether my act is one of forgiveness.
If you order a drink in my bar and I say, "This one's on the house," I am forgiving the price of the drink due me. Where is the injustice?
The critical point here is that, for me to forgive, I must renounce a just claim to something due me. As everyone who says "I can't forgive a murderer who murdered someone I don't know" argues, something due someone else cannot be forgiven by me (unless I have authority over the other person, as a father, say, or an elected official).
Think about what happens when one person wrongs another. Harm is done to the person wronged, of course, but harm is also done to the wrongdoer, interior moral harm if nothing else. Harm may also be done to the larger group (the family, the society) within which the two people are related. Assuming the wrong done is sufficiently grave, the wronged person has some reparation due, as does the larger group, and the wrongdoer has some punishment due him.
The wronged person can choose to forgive the reparation due him. The larger group can choose to forgive the reparation due it. Can the wrongdoer choose to forgive the punishment due him? Actually, the way I've set things up, he can, but since no one can force another to accept forgiveness, he's likely to hear, "No, no, we insist!"
I think the interesting question is, can the wronged person or the larger group forgive the punishment due the wrongdoer? Again, you can only forgive what you have the authority to forgive. Just to fix ideas, let's think of the case where the wrong is a crime and the larger group is a society. Then, if the crime harms the common good and the only way to repair that harm is by punishing the wrongdoer, the society really doesn't have the authority to forgive punishment. That is, it might have the legal authority, but it is unjust for a society to forgive just punishment at the cost of harm to the common good.
But it's not unjust for a society to forgive just punishment if doing so doesn't harm the common good. Equally, it's not unjust for an individual to forgive the punishment of another (or to advocate forgiveness, if he doesn't have direct authority) if doing so doesn't harm the relationship between the two.