Well it is true that the English killed many French prisoners, but they would only have been a small minority of French casualties and when you are that badly outnumbered in a battle the last thing you need is a hostile group of prisoners you dont have enough men to guard left in the rear. In any case the French trampling their own men is no different from the military incompetence with which they generally responded to the English threat during that particular campaign. The English either had better soldiery and used it more professionally or otherwise they simply exploited the weaknesses of their opponents.
One thing we did with medieval history is look at witchcraft. Other factors aside such as the reformation which caused much of the "witch-hunt" mentality, i saw a program on TV which claimed that ergot poisoning drove people mad. Ergot poisoning occurs when especially cold weather causes a fungus to grow on rye crop which is then eaten. Supposedly the same thing caused the collapse of Peter the Great's army as he was preparing to invade Europe and he had to abandon the planned campaign. Ergot poisoning wouldn't be the sole reason for the witch hunts, but they may be an important factor.