This was one of the scenes that struck me when I first watched Waterloo (1970). It gives such a fascinating insight into Wellington's character.
Andrew Roberts puts it best: "By contrast to today, when an enemy head of state constitutes a legitimate military target, Wellington refused an artillery officer under his command permission to fire his battery at Napoleon.” (Waterloo. HarperCollinsPublishers, 2005. p.15.) Christopher Hibbert quotes Siborne, a Waterloo historian who served as a lieutenant in the 9th foot, providing more detail:
“An officer of the artillery came up to the Duke, and stated that the had a distinct view of Napoleon… that he had the guns of his battery well pointed in that direction, and was prepared to fire. His Grace instantly and emphatically exclaimed, “No! No! I’ll not allow it. It is not the business of commanders to be firing upon each other.” (Hibbert, Christopher. Wellington, A Personal History. Addison Wesley, 1997. p.212.)
Tactically, eliminating Napoleon before the battle ever began would have made sense. Without Napoleon, the battle might not even occur. Even if the Marshals tried to rally and fight for Napoleon's son, French morale would be wrecked. Wellington himself estimated that Napoleon's presence was worth 40,000 men; with his death would go all the personal charisma he could muster. (Hibbert, Christopher. Waterloo: Napoleon’s Last Campaign. Mentor Books, 1967. p.141.)
Consider that French sharpshooters particularly targeted Lord Nelson during the Battle of Trafalgar and killed him. (The Great Commanders, "Horatio Nelson.") There was precedent for giving the order, yet Wellington chose not to. I've seen it argued that Wellington was trying to kill Napoleon, and yet this choice makes that point of view very difficult to defend.
I cannot say precisely what drove Wellington to make this decision, but I suspect it had to do with his ideals of honor, his commitment to being a gentleman. Perhaps there were political considerations as well, but if Siborne tells it accurately, the instant response belies such careful thinking. Siborne doesn't say that he deliberated, but that he instantly and emphatically denied permission. I don't discount a flash of genius on Wellington's part, I just find honor the likelier explanation.
Wellington was a gentleman, from his hat to his boots. This incident is merely one potent example of that fact.
