يَٰٓأَيُّهَا ٱلنَّبِىُّ قُل لِّمَن فِىٓ أَيْدِيكُم مِّنَ ٱلْأَسْرَىٰٓ إِن يَعْلَمِ ٱللَّهُ فِى قُلُوبِكُمْ خَيْرًا يُؤْتِكُمْ خَيْرًا مِّمَّآ أُخِذَ مِنكُمْ وَيَغْفِرْ لَكُمْ ۗ وَٱللَّهُ غَفُورٌ رَّحِيمٌ

Releasing the prisoners at the battle of Badr was a strategic error. But, for the prisoners themselves, this amounted to giving them a new lease of life. This meant that the people who had become liable to be killed due to their opposition of the Truth, were given a further opportunity to re-think the Islamic call and their improper behaviour towards it. This respite opened up for them the door of self-rectification.Now one consequence of this event could have been that the flames of revenge flared up in the hearts of the prisoners due to the defeat they had suffered; they could have become eager to avenge the ignominy and loss they had suffered on the payment of ransom. In this case, they would have repeated the same wrong action as a result of which they had become liable to be seized upon by God. They would have expended their energy in opposing Islam, the result of which action would have been destruction in the world and punishment in the Hereafter. The second possibility was that they could have thought over the extraordinary happenings on the battlefield of Badr, i.e. why did it happen that, in spite of having inferior and fewer armaments, the Muslims were clearly victorious? This clearly indicated that God was in favour of the religion of the Muslims and disfavoured that of the Quraysh. If this second line of thinking developed, it would make them change their previous behaviour and adopt the religion which they had not earlier been able to adopt and in this way become eligible for God’s reward in this world and also in the Hereafter.History shows that there were a number of individuals among the Quraysh in whose hearts this question had arisen and sooner or later they joined Islam. ‘Abbas ibn ‘Abdul Muttalib had embraced Islam when he was a prisoner. Some others also joined the Islamic circle later. These people lowered themselves in the eyes of their own prejudiced group, but they honoured themselves in the eyes of God. They suffered worldly harm, but became possessors of the benefit of the Hereafter. On the release of the prisoners, the Muslims had the suspicion that the latter, instead of being grateful for the Muslims’ magnamity, would engage in conspiracies, adopt destructive ways and create impediments in the way of Islam. But the Quran did not give importance to these suspicions, because a movement launched for the cause of pure Truth is not an ordinary movement. It is a Divine affair. God Himself is behind it; and nobody has the power to fight God.

— Source: Tazkirul Quran (Maulana Wahiduddin Khan) via Quran.com (8:70)