It sounds odd but killing prisoners (specifically nobles) was seen as a very bad thing during the high-medieval age. For multiple reasons; however the primary one is ransoms. Ransoming enemy nobles was big-business and the main reason knights were so keen to get into battle. A relatively poor land-owner could make a fortune by capturing a rich opponent.there should be no execution penalty if you are currently at war with them. its fine to kill them in battle but not after? lol wut?
After the battle of Agincourt the English King demanded the French nobles be executed as there was simply too many to hold. Nor did he much wish to profit from them. The English Knights refused to do it and eventually it fell on the longbowmen to perform the executions. It didn't do him any favours with the 'relationships' of the English or French nobility.