Hmmm... while it's true that a knight was more valuable alive than dead, I fear that the ransom itself was a great motivator as much as a sense of chivalric courtesy.
Did Henry V act without honor when he ordered the execution of the French prisoners at Agincourt? The French, who greatly outnumbered the English were attacking with their 3rd wave, and the English had so many French prisoners that Henry feared they would rise up while the English line was preoccupied with the battle. Henry acted to save his own men, but had many of the French peerage executed in order to do so.
In pitched battle, which was quite rare in the medieval era, despite what movies would have us believe, I think just like today, you kill who you need to kill to stay alive and save your brothers in arms. If that guys the Viscount of Umptysquatshire, and he's about to kill you, you're going to stick a poleaxe in his throat if you can and worry about capturing him afterwards.