It is absolutely not the case that killing was prohibited or looked down upon in medieval combat. While there was a strong incentive to take hostages for ransom and/or extracting political concessions (if the captive was important enough), plenty of soldiers were killed outright. It might happen by accident (no such thing as "shooting to wound," whether with a gun or a crossbow) in the heat of battle, or it might be on purpose. Civil wars and interpersonal strife between families often led to situations where rival claimants to a throne were butchered immediately upon capture. The Wars of the Roses were notorious for such behavior. Military necessity might also lead an army to murder its prisoners, regardless of the potential ransom money. This would not be considered a violation of the customs of war, but a grim reality of combat in the period. Men might also be killed in the effort to take an individually important prisoner. Many of King John II's bodyguards were cut down by the English at Poitiers, as they were more focused on taking the king than on taking as many prisoners as possible. While ransom was important in the culture of the day, killing outright was not prohibited.