Evaluating the real life historical behavior of knights in comparison to this ideal is problematic and complicated to say the least. While there were many Medieval and Renaissance knights who were considered to be paragons of chivalry by their country or even internationally, very often A Hero to His Hometown would be in effect and opinions would depend on who you asked. To take some examples from The Hundred Years War, Henry V of England is often held up as a patriotic hero for his bravery at Agincourt, but the fact that he ordered the execution of French prisoners in the middle of the battle and subsequently committed what might be called atrocities during the siege of Rouen casts a darker light on him. You also might find that some French knights would have considered Edward III and Edward the Black Prince to be worthy opponents, but the French peasants whose lands they ravaged would have had a less charitable opinion of them. When such characters are cast as the good guys in a modern work they will often get a Historical Hero Upgrade that removes these morally grey aspects of their deeds.
There is also a certain amount of Values Dissonance because the modern popular understanding of chivalry prevalent since Victorian times is to some extent a distortion of what it meant in medieval times to the knights who practiced it. Today the word is usually associated with courtly manners, mercy and fair play in warfare, and protection of women and the weak, but we must not forget that first and foremost it was an ideal of behavior for aristocratic warriors meant to encourage military prowess. Courtly Love for example, while genuinely important, was often described more as an incentive for a knight to try to distinguish himself in tournaments and warfare than as an end in and of itself. Modern readers of chivalric literature are often surprised at its sheer emphasis on the ability to kick ass and perform awesome feats, as opposed to the finer points of etiquette. In terms of warfare it was mainly concerned with relations between social equals, such as the rules for knights ransoming each other, and did not apply equally to common footmen or enemy noncombatants. Indeed, it was by no means as expansive as The Laws and Customs of War, and in comparison was a loose set of guidelines for the warrior elite's martial ethic as opposed to some kind of law agreed to by international treaty. Therefore, the French felt justified in flying the Oriflamme banner signifying no quarter when the nation was threatened, and it was also considered within the norms of warfare to butcher the defenders of a town or castle that was taken by storm after it had stubbornly refused to surrender. Those that surrendered quickly were treated leniently, while those that surrendered after a long siege would lose much but be allowed to live. Since sieges were so damaging to both the attacker and defender, the threat of holdouts being put to the sword was viewed as potentially saving more lives by discouraging defenders from resisting to the bitter end. Even within these surprisingly broad guidelines, there were many knights who were essentially unscrupulous mercenaries or criminals dignified by horse, armor, and a noble title.
All this should not be taken to mean that there were no knights who even today could be seen as examples of good behavior or at least honor in battle, as the Real Life section hopes to show. Perhaps it is best to describe Chivlary as an ideal to which most knights aspired, at least publicly, and conformed to in varying degrees.
