Аль-Исра 17:33

The killing of anybody without the sanction of the Islamic law is totally prohibited (haram). One who is killed without the justification of the Islamic law must be regarded as having been wronged. In view of this, the surviving kin of the deceased have full rights over the killer; they may avenge the murder as per the law; or they may let the killer go, after taking compensation from him; or they may forgive him altogether. According to Islamic law, the real claimants are the surviving kin of the dead person and not the government. The duty of the government is only to help the surviving kin to enforce their will. Murder is such a terrible crime that it is said in a tradition of the Prophet Muhammad that the killing of a human being is more undesirable that the annihilation of the whole world. In the eyes of God, this principle notwithstanding, the surviving relatives of the murdered person have no right to commit any excess on the killer in the avenging of the murder. They may not, for instance, disfigure the killer or kill one of his companions in his place, etc. If the surviving relatives of the killed person commit any excess in avenging the murder, in this case the government may step in to prevent this, in exactly the same manner as it would have assisted them in securing their right to retributive justice, or qisas. This reveals the spirit of Islamic law: if a person, howsoever oppressed, wants to take revenge, he is allowed to do so only in proportion to the degree of oppression he has suffered. He is in no way allowed to take any action in excess of this.