Anti Islamists allege here that the prophet killed Nadr because of his anti Islamic poetry. What the convienently fail to mention however is that in the Battle of Badr, Nadr joined the enemy army! They make a big deal out of his killing, but obviously Nadr joined the enemies of Islam to do battle with Muslims, and to kill Muslims. This is what happened during the battle, and as such he risked his own life by taking up arms against the Muslims.
After the battle, out of the 72 captives, only 2 were executed, one of them being Nadr b. al-Harith. Here is the account:
" It would be in the fitness of things to say a few words about the magnanious treatment that was accorded to the prisoners of war by Muhammad (peace be upon him) and his companions. This noble attitude which they showed can be fully appreciated if we review it in the context of the circumstances which led to the war. The Prophet and his companions had endured for full fifteen years unspeakable insults and injuries at the hands of the Quraysh of Mecca so much so that they were obliged to bid goodbye to their native place and seek shelter into a far-off place. The Meccans who were thirsty for their blood did not allow them to lead a life of peace even in their new abodes. They fell upon them with all their forces in order to exterminate them root and branch. Fate, however, decided otherwise and they were defeated by a small army of the Muslims. Amidgst such feelings of bitterness the Prophet (peace and blessings of Allah be upon him) remained calm and self-possessed. No atrocity was perpetrated upon the prisoners. Out of the seventy-two captives only two were executed, viz., al-Nadir b. al-Harith and Uqbah b. Abi Mu'ayt who were notorious for their unrelenting hostility towards the Muslims. The rest of the captives were treated with utmost kindness and consideration. "Blessings on the men of Medina," said one of these in later days, "they gave us wheaten bread to eat when there was little of it, contenting themselves with dates." It is not surprising, therefore, that some of the captives, yielding to these influences, embraced Islam and were therefore immediately set free. The rest were kept for ransom. But this was long before Quraysh could humble themselves to visit Medina for the purpose. The spell of kindly treatment was thus prolonged and left a favourable impression on the minds of those even who did not at once go over to Islam.
The ransom of each prisoner varied with his financial position, ranging from one thousand dirhams to four thousand. The poor who could not afford to pay were set free without any compensation. Those who could read and write were given the charge of small children. Each one of them had to teach ten of their wards and when they became proficient in reading and writing, their instructor was granted liberty. This condition of securing freedom throws a good deal of light on the value which Islam attaches to learning."
(Source: The Life of Muhammad PBUH by Abdul Hameed Siddiqui, p.185-186, Islamic Publications LTD.)
The two men, were executed because of their unrelenting hostility towards the Muslims. However, whats interesting, is that out of all the prisoners of war, only 2 were executed. Moreover, these prisoners were treated with the utmost kindness. So much so that history records many of their conversions to Islam. Examples of the kindness mentioned above include the fact that the Muslims gave them the better thing to eat, while they contented themselves with dates.