It’s part of Wild Bill Hickok’s legend. The acting Ellis County (KS) sheriff was trying to corral Bill Mulvey, who was shooting up Hays City. Mulvey got the drop on Hickok and threatened to kill him. Wild Bill then looked beyond his adversary and called out, “Don’t shoot him in the back boys, he’s drunk.” When Mulvey turned to see who was behind him—nobody was—Hickok pulled his pistols and fatally wounded him.
A great story, but Hickok biographer Joseph Rosa says there’s little evidence it happened that way. But he did kill Mulvey.