why did sherburn shoot the drunk man?

In Chapter Twenty-One, a man named Boggs rides into town. He is a drunk who comes in each month and threatens to kill a man, but never actually harms anyone. This time, he is after a Colonel Sherburn, the wealthiest man in town and a storeowner. Boggs stands outside the store and screams insults at the Colonel. The Colonel comes out of his store and tells Boggs that he will put up with the insults until one o'clock and after that he will kill him if Boggs utters even one word. Boggs continues relentlessly, and at exactly 1pm, the Colonel appears and kills Boggs on the spot.