You have probably heard or read that to gain internal peace you just have to forgive the perpetrator of whatever injustice had been inflicted on you. But this is wrong. It leaves the question of justice unresolved.
So how do you gain peace? Sometimes the perpetrator has died. Sometimes the injustice goes on. Often there has been no repentance on the part of the perpetrator and even more rare is the required restitution needed to heal the relationship that has been damaged or broken.
But we can gain the peace of Christ. The Scriptures say, "Vengeance is mine, saith the LORD. I will repay." Also, there is a Judgment Day coming where all injustices will be addressed and made right. There is nothing more sure than this for God is just, His justice shall be satisfied, and His law will be upheld.