Ya ayyuha allatheena amanoo kutiba AAalaykumu alqisasu fee alqatla alhurru bilhurri walAAabdu (surah Baqarah: 178)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(2:178) O Believers, the law *176 of retribution has been prescribed for you in cases of murder; if a free man commits a murder, the free man shall he punished for it and a slave for a slave : likewise if a woman is guilty of murder the same shall he accountable for it. *177 But in case the injured brother *178 is willing to show leniency to the murderer, the blood money should he decided in accordance with the common law *179 and the murderer should pay it in a genuine way. This is an allowance and mercy from your Lord. Now there shall be a painful torment for anyone who transgresses the limits after this. *180 O men of understanding.

Yaaa ayyuhal lazeena aamanoo kutiba alaikumul qisaasu fil qatlaa alhurru bilhurri wal`abdu bil`abdi wal unsaa bil unsaa; faman `ufiya lahoo min akheehi shai`un fattibaa`um bilma`roofi wa adaaa`un ilaihi bi ihsaan; zaalika takhfeefum mir rahbikum wa rahmah; famani` tadaa ba`da zaalika falahoo `azaabun aleem

*176). Retribution, that is, blood revenge, is based on the principle that what a person has done to others ought to be done to him. This does not mean that the murderer should be put to death in exactly the same manner as he killed but that the murderer should be subjected to the same act, i.e. killing, as that to which he subjected his victim.
*177). In pre-Islamic Arabia people tried to take blood revenge upon the murderer's family and tribe, and the retaliation corresponded to the value placed on the blood of the victim. Their desire for revenge was not quenched merely by putting the murderer to death. They preferred to put to death tens and even hundreds of people to avenge the one life they had lost. If a respected member of their tribe was killed by an ordinary member of another, it was not deemed enough to put to death the actual murderer. They preferred to kill a man of the murderer's tribe equal in standing to the victim, and even several members of the murderer's tribe. However, if the victim was a man of humble standing from another tribe, and the murderer from their tribe happened to be a man of high standing, they were unwilling to permit the execution of the murderer.
This attitude is not confined to the Ignorance of that bygone age. Even today those nations that are supposedly the most civilized will often proclaim, officially and quite brazenly, that if one of their citizens is killed they will execute scores of the killer's compatriots. In addition we often hear that to avenge the murder of one person a large number of hostages belonging to a subject nation have been shot dead. One of the 'civilized' nations of the present century subjected the whole Egyptian nation to blood revenge because one of their officials, Sir Lee Stack, was killed by an Egyptian. The courts of justice of these so-called civilized nations have been known to refrain from passing the death sentence on convicted homicides when they happened to be members of the ruling nation while their victims belonged to the subject nation. It is iniquities such as these that God seeks to end by means of the directive contained in this verse. What God says here is that the killer ought to be put to death irrespective of his status and that of the victim.
*178). The very use of the word 'brother' in this context suggests that as a general rule one ought to incline towards leniency. Despite the bitterness felt towards someone who has shed the blood of, say, one's father, the murderer is still one's brother by virtue of being a member of the human family. Hence if one who has been wronged can overcome the vengeful spirit aroused by his erring brother's deed, this attitude of forgiveness will be worthy of his humanity.
This verse also makes it clear that according to the Islamic penal law the question of homicide can be settled by the mutual consent of the two parties. It is the prerogative of the heirs of the victim to forgive the murderer, and if it is exercised not even a judge has the power to insist on carrying out the death sentence. In such a case, however, as the following verse mentions, the murderer will be made to pay blood money.
*179). The term ma'ruf occurs quite frequently in the Qur'an. It refers to conduct which is reckoned fair and equitable by the generality of disinterested people. The generally accepted usages and customs of life are called 'urf and ma'ruf in Islamic terminology, and they are considered valid in all those matters not specifically regulated by the Shar'iah.
*180). Excess might consist of trying to avenge the blood of the murdered man even after his heirs have settled the matter and received blood money or of efforts on the part of the murderer to delay the payment of blood money thus repaying the heirs of the victim with ingratitude for their kindness and goodwill.