>>12023
Most killings happened after the battles when one side broke and fled. Grossman writes about this too. This type of killing has two components:
1. the killer doesn't see the face of the fleeing enemy
2. the ancient hunting instinct kicks in, chasing is a very thrilling experience which makes easier to murder
The best chasing unit was the cavalry which is usually considered a noble unit (however lots of man-at-arms - who are often confused with knights - were commoner).
Also I think nobles in the described situation you quote were more of a killers through their commands they gave.

>Until 17th century armies (at least in Europe) had much more professional/mercenary composition, and masses of poor serfs weren't active participants of most wars.
Untrue. The usage of mercenary bands were very limited by the lack of money and the suitable social strata up to the 13-14th century. After that it was still very varied in different regions but of course the paid soldiering became the norm. For example in England during the Wars of Roses parts of the troops still came from the feudal levy (maybe even they got some moniez) on the other hand in the Italian Wars mercenaries were extremely popular. Probably such factors as scope of conflicts and the partaking sides also influenced this.