They killed peasants and mercenaries instead. The knights they took hostage for ransom.

My understanding is that in battle, there were very few knights in full plate - it was very expensive, required training, ie. tended to be nobles. So while swords were effective against the peasants, knights were usually captured alive and ransomed.

if someone can afford plate armor, they can probably afford a decent ransom

edit: beaten to it, that's what i get for loading a whole bunch of tabs when i first get into work...