Justice, injustice and forgiveness all fit together in this way. When we've been treated unjustly and something that was permanent let's say there's a murder of someone's child, you cannot reverse that. But what you can reverse is the effects of that on you and on your relationships, on your family. One cannot bring the child back. But what someone can do is get rid of the resentment that can lead to depression, anxiety and hopelessness. Because forgiveness targets that particular malady if you will which is a resentment that won't leave.
Also when a person begins to exercise the virtue of forgiveness they realize that there are many virtues that are worth appropriating. There are many virtues that are worth developing and people naturally tend to bring justice alongside the quest for forgiveness. You see forgiveness is not a weak response. You stand in the pain and give goodness to the one who hurt you. And at the same time you ask something of the one who hurt you. You cannot reverse the injustice but you can ask for justice going forward and be healed of that which literally can kill a person.