The ability of upper classes to murder people in lower classes actually varied a bunch by time and place. The Holy Roman Empire had a court system that did make it feasible for commoners to press complaints against nobles, and to win; it aimed explicitly at resolutions everyone could live with rather than an objective guilt or innocence. Really unchecked lordly power is more an early modern thing than a medieval one. There were plenty of rotten lords before that, but in times of smaller populations and fewer resources, there were practical limits on how tyrannical you could be before your seeds and neighbors decided you were too much trouble and did something to you to make you stop. Larger populations and more stuff made the potential power gradient worse, and absolutist ideologies encouraged using it.