Upvote:2
The fact the king holds the right of life and death is hard to challenge, even after Magna Carta. Kings were supposedly judges, conducts of divine will, Destiny manifested etc etc all in a marvelous package. Thus, the King could sentence people to death, if for good reasons. John the first was notorious for beeing, well, an annoying king, that often abused his position, hence the Magna Carta. For others kings, if they were "good" enough, they wouldn't be as challenged.
At that time in history, pitched battles were the norm, not skirmish. Killing a visiting commander wouldn't be honorable, nor accepted by many. If you consider the war history, the French king was, at one point, held hostage by the English. This king was at one point freed, but before the ransom was paid. He went back to the English to stay until the ransom was paid. So, there's that. The same can be said for challenges, refusing a challenge was dishonorable.