The accounts of how the massacre exactly happened are contradictory. One simply says that the English killed all of their prisoners except for a few deemed "illustrious" enough to spare. This does not specify exactly how the killings were administered. Another describes how the English weren't really excited about the prospect of losing their ransom money, so Henry placed one of his esquires in command of a special unit of archers to actually do the deed. None of the sources give us any detail as to the number of prisoners at this time of the battle, but modern estimates have come up with numbers as high as three thousand. If this is even somewhat close to the real number, then it explains Henry's desperation to have them all killed, but not why the French prisoners passively accepted their slaughter.

In solving this mystery, we must turn to a third account, that of the French knight Ghillebert de Lannoy, who was captured at Agincourt. After being wounded, he was seized by the English and locked in a local home with a handful of other French soldiers. When Henry ordered the execution of the prisoners, the English set the house on fire. de Lannoy barely managed to escape the house, but was captured again by someone who recognized his value as a prisoner. By this time, the moment of desperation had passed, and the English were content to take prisoners for ransom again.

If de Lannoy's account is true, and the English did in fact herd at least some of their prisoners off the battlefield and into separate groups, it goes a long way towards explaining exactly how the massacre of the prisoners occurred. Some, and possibly the majority, of the prisoners would still have been closer to the front line of the battle, where they had been taken. After this had been carried out, Henry's special execution squad could easily run back and slaughter the groups of prisoners that had been marched away from the front lines previously. The French prisoners would have all been disarmed and demoralized. Many were likely wounded, as de Lannoy was. Even if the French prisoners thought about fighting back as they were being slaughtered, they would not have much ability to do so.

Henry's decision to slaughter the prisoners was counter to ideals of war and chivalrous knightly behavior, but contemporary sources seem to have recognized the necessity of his decision. The English had previously executed wounded French soldiers at the Battle of Crecy in 1346. The Portuguese army also massacred their prisoners when the situation seemed desperate at the Battle of Aljubarrota in 1385. Curiously, there was a force of English archers present at Aljubarrota fighting for the Portuguese. It may have been the case that they were the ones who executed the prisoners on this occasion as well. While executing prisoners taken for ransom was not common, neither was it considered completely out of the question if the situation called for it.