Rabi' bin Anas (may Allah have mercy on him) narrates from Abu al-Aliyah (may Allah have mercy on him) regarding "Then whoever transgresses" [Surah al-Baqarah: 178]: that is, killing after taking the blood money (diyah). "He will have a painful punishment. That is a concession from your Lord and a mercy" [Surah al-Baqarah: 178]: its meaning is that for the Jews there was no blood money in the case of killing; there was either pardon or retaliation (qisas), and for the people of the Injil (the Christians) there was only pardon, whereas Allah has given this Ummah all three concessions: if you wish, kill in retaliation; if you wish, take the blood money; if you wish, forgive. "And there is life for you in retaliation" [Surah al-Baqarah: 179]: that is, Allah has made retaliation a means of life, in that if someone is killed in retaliation, others will take heed from it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15881