But the nature of the feudal system leaves no one unaccountable, and the commoners would likely seek compensation or enforcement through the other lords if he would not.
It's a good call not layering on modern conceptions, but to understand the period you also need to study the laws, government, customs and culture, not just accounts of brutality and battles, or you're looking at the situations through a peephole.
The times could be harsh no doubt, but don't go overboard or base your views only on the harsh parts.
There are a couple of problems with this argument. First of all, in most of medieval Europe there was nothing like what we would consider a strong central government. Even in England, one of the more centralized of feudal states, the king's power still rested to a great extent on the cooperation of local vassals. In places like France, or the HRE, authority was far more decentralized, with individual lords ruling small towns, and even an entire group, the imperial knights, basically acting as one person countries. The feudal lords of each area were, in effect, the law. They decided how it would be enforced and they had the military power to back up their decisions.
There are numerous historical examples of nobles abusing their authority. Jeanry has cited a couple of them. Rather than being unusual exceptions, given the disparity of power between lords and subjects, I feel that it would be reasonable to assume that such abuse was more common than has been documented, not less. A commoner being able to appeal mistreatment to another lord, ie his lord's lord or the king, would have been unusual. Unless the commoner was A. wealthy or B. had the support of a powerful patron (such as the Church) , the likelihood of him achieving redress against his lord would have been extremely low. I would like to see some historical examples of any cases where those factors were not present. I believe the historical evidence we have strongly supports an assumption that a knight or other noble, could kill a peasant or two, and suffer no retribution, as long as the person he killed was one of his own subjects and not that of another lord. And note, I'm am writing specifically of the peasants who made up the majority of medieval populations, not of townpeople who form a special case.
The main recourse of peasants was rebellion if abuse was systemic. However, if you examine what happed after various peasant rebellions, the main concern of the aristocracy was not to redress grievances, it was to slaughter the peasants for daring to rebel in the first place.
Incidentally just so you know I'm not just spouting off the top of my head, I have B.A. & M.A. in history and am A.B.D. on a PhD. I am not an expert on medieval history, but I have studied it and taught it to undergrads -- for what that's worth, which may not be much <img src="/forum/images/icons/smile.gif" alt="" />. Everything that I have seen though, does essentially support the view that the medieval aristocracy was, what we would consider "above the law," when it came to dealing with the peasantry.