Because they have taken human life illegally, in a deliberate and premeditated manner. That sounds like the definition of 1st degree murder to me. It wasn’t legal at the time either. I doubt it has been been legal at anytime since the judicial duel was eliminated as a means of litigation.
Oh, you mean why should it be considered murder? Because the person who is challenged is under a social compulsion and thus cannot give uncoerced consent; either they risk death or their social standing suffers certain death.
It has often been argued that the objective of the early American duel wasn’t to kill your opponent; that could have adverse social consequences even if you were never charged with anything. The objective was to force your opponent to back down without a fight, thus proving that you are the Better Man. For example, Abraham Lincoln was once challenged to a duel. By custom, the person who was challenged was allowed to choose the weapons. He wisely chose broadswords. His opponent wisely backed down as he didn’t want to face a 6’4" man who split logs for a living with broadswords. Thus, Lincoln won.
As a contrary example, the Burr-Hamilton duel killed Hamilton and ended Burr’s political career. Thus, no one really won.
