There is no hard and fast answer to this question. Medieval warfare is difficult to define and as chaotic as warfare in any other era. Individual commanders had their own ideas about how to treat enemy combatants, or made decisions based on the circumstances.

Before he died Richard I pardoned Pierre Basile and according to some accounts even paid the boy a stipend. Basile was reportedly executed after Richard’s death by a mercenary.

Sticking with the English army the longbow was a weapon which would allow a peasant to kill a nobleman and this has been credited with producing an egalitarian spirit amongst the English ranks. Geoffrey the Baker reported Edward of Woodstock, the Prince of Wales, referring to his archers as “descended from the blood of… Kings of England…” ahead of the Battle of Poitiers in 1356 which is very high praise indeed. For fighting in close quarters, longbowmen might use a small sword or a hammer to enable them to pick out week spots in an enemy’s armour and reportedly after a battle they would be allowed to patrol the battlefield putting enemy soldiers out of their misery. In this sense there were no issues on the English side with a peasant killing noblemen and although there are reports of longbowmen being held in contempt by the French, this is probably more to do with their general effectiveness rather than their ability to stick it to the aristocracy.

As for taking prisoners there can be no doubt that an imprisoned nobleman is a valuable asset as long as the cost of housing and feeding them is not greater than the potential ransom. I have found no evidence that anyone in fear of their life would be expected to try and capture an enemy nobleman rather than kill them. At Agincourt in 1415, Henry V took the unusual step of having a large number of prisoners executed (possibly over a thousand), probably because he was concerned that they might be able to overwhelm his small army. It is reported that English knights refused the order and so a company of longbowman carried out the King’s wishes.

As /u/CopperRoyce has alluded to, a soldier’s fate is most likely to depend on whether he wins or loses the battle. I’m not aware of any soldier on the winning side being punished for killing an enemy nobleman and retributions against the losing side were always likely regardless of individual soldier’s actions.

Apologies for not being able to give a definitive answer, but hopefully this gives you an idea of the complexity of the subject, the wide variety in medieval warfare and the role that circumstance has in influencing commanders’ decisions. Hopefully there will be plenty of other answers from users familiar with other areas of medieval warfare.

In Japan, a commoner killing a high ranking enemy combatant was the number one most reliable way to rise up the ranks for hundreds of years and actually represented the main form of social mobility in Japanese society.

The tradition of cutting off the heads of a high ranking enemy that they had slain and subsequently bringing it back to show their own lord was fairly standard. They would then have been slowly elevated, with many peasants becoming part of the privileged samurai class.

Being elevated to a higher social position would also entail their associates, rising, with families becoming clans/houses and elevated far enough, they would have become lords in their own rights, with even extended family being elevated and forming subsidiary branch houses with their own vassals and holdings.

The entire system of rewards, identifying the kills, division of spoils, competition on the battlefield for heads and corpses, etc. is very extensive and grew organically to its peak during the Sengoku Period, where most soldiers in Japanese armies were semi professional commoners, all competing and constantly looking for high ranking samurai to behead and take home for a big payout.