Factual error: When the dead cowboy is brought into town and Little Bill is getting briefed, one of his deputies remarks that "Witherspoon" will not sell them any more 30-30 shells unless they pay. The movie takes place in 1881, but the 30-30 cartridge wasn't developed until circa 1895. (01:36:45)
Factual error: During the confrontation between Little Bill and English Bob outside the barber shop, all the Deputies cocked their guns - for emphasis. Then Mr. Beauchamp reaches into his bag to show that he only has books, the Deputies cock their guns again. With the rifles they have this would eject the shell that was in the barrel and cycle a new one. It doesn't.
Factual error: English Bob is shooting the pheasants from the train. The film is supposed to be set in 1881. Ring neck pheasants are not native to the US. They were imported from Shanghai, China and not stocked east of the Rockies until 1898.
Factual error: The tension rod in Ned's house is plastic - wrong for the time.
Chosen answer: I think Little Bill takes offense to this sort of thing taking place in his town on his watch. He tells Mr. Beauchamp while he is writing his biography something like, "I do not like assassins, or men of low character."