Muqatil bin Hayyan (may Allah have mercy on him) said concerning this verse, "Retaliation (qisas) is prescribed for you in the case of those who are killed" [Surah al-Baqarah: 178]: Before Islam the Arab tribes used to fight one another; then they became Muslims, but some vendettas remained between them. So they began to demand that a free man be killed in return for a slave and a man in return for a woman, and they even swore oaths on this. But Allah Almighty revealed this verse, and then all of them submitted to it and were content with it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15891