All perfect praise be to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Transgression is treating others unjustly and abusing them verbally or physically, and Islamic Sharia prohibits such a thing. The Law-giver gave the transgressed person the right to transgress against the transgressor likewise. Allah, The Almighty, said: " If then any one transgresses the prohibition against you, Transgress ye likewise against him. But fear God, and know that God is with those who restrain themselves."{Al-Baqarah/194}. Allah also said : "The recompense for an injury is an injury equal thereto (in degree): but if a person forgives and makes reconciliation, his reward is due from God: for (God) loveth not those who do wrong. But indeed if any do help and defend themselves after a wrong (done) to them, against such there is no cause of blame. The blame is only against those who oppress men and wrong-doing and insolently transgress beyond bounds through the land, defying right and justice: for such there will be a penalty grievous." However, Allah stimulated people to show forgiveness where He continued the above verse by saying: " But indeed if any show patience and forgive, that would truly be an exercise of courageous will and resolution in the conduct of affairs."{Ash-Shura, 40-43}.
Therefore, it is preferable that the transgressed person forgives and excuses and his recompense is due from Almighty Allah; however, he may not do so, and in this case he is allowed to claim his right in the life of this world or in the Hereafter, but then he would have missed that which is better. This is because, in principle, a Muslim should forgive the one that oppressed him so that Allah would treat us in that exact manner for He said: " let them forgive and overlook, do you not wish that God should forgive you? For God is Oft-Forgiving, Most Merciful."{An-Nur/22}. And Allah knows best.