Let's pretend for the moment that you're the knightly sort. You've just been attacked by bandits, in particular a bandit chief, who makes short work of your companion. Yet the bandit chief doesn't kill your companion; as soon as it is clear that he/she can't defend him/herself anymore the chief stops attacking.
After inflicting nearly a mortal wound on this bandit chief, he then drops to his knees and shouts "I yield!"
Admittedly, you are a knight that is not exactly new to this behavior. At the back of your mind you know this chief may act like all the others and merely try to attack you again once he recovers enough to do so. Yet, as a knight, it is not honorable to slay someone while they're basically begging on their knees and not defending themselves. This is also a bandit that just spared the life of your companion as well.
If only you had some actual rope to tie up the bandit and escort him to the nearest prison like you've seen both sides of the civil war do. But no, you're only options is to not attack and let him run around in circles crying before he gets around to attacking you again or hope your companion finishes him off for you.

And on that note, why are all the special horses in this game only ones you can get if you are of questionable morality. Where is my rideable white elk, dammit.
/rant over