What is the wisdom behind Allah's severe reproach to the Prophet, peace and blessings be upon him, regarding the prisoners of Badr, even though it was his own ijtihad (independent reasoning) and not preceded by a divine ruling? And when did the divine permission to take captives after their ransom begin?

The verse "It is not for a prophet to have captives until he has made widespread slaughter in the land" was revealed on the Day of Badr. It is a reprimand from Allah to the companions of the Prophet (peace be upon him) for taking captives before achieving widespread slaughter. The reprimand was directed at those who suggested taking ransom, as the Prophet (peace be upon him) did not order it. The permissibility of taking captives after this reprimand is indicated by Allah’s saying: "So when you meet those who disbelieve, strike [their] necks until, when you have inflicted widespread slaughter upon them, then secure their bonds, and thereafter [either] favor or ransom until the war lays down its burdens."