In a feudal setting the greatest crime is to betray your lord. Failure is nearly as bad. So what happens when you're the sworn guard to your lord, and he dies? When you get home and face his widow and family, what then? Even if they forgive you, you've still failed. Your oath has been broken. How do you pick yourself after that?
If I need an excuse for a broken, fanatic killing machine of a protagonist, suffering from PTSD, determined that he will not fail again, or at least he won't outlive failure this time, here it is. Of course this links to some other themes - death and family. Which by some coincidence come next on my list of Write What You Know.
 Unfairly treating your vassal is equally bad. However as the rights and privileges increase as you move up the ziggurat, and the responsibilities increase as you move down, plus judging it happens at the top, for some reason it rarely happens that way. Who would have thought?