The Hair-Trigger Kid by Max Brand

Out of the town, as he had come into it, the Kid rode most leisurely. No one halted him; and only Tommy Malone asked him to have a drink.

He refused the drink, with apologies for the demands upon his time which made it impossible for him to linger, no matter how he wished to. But when he got farther down the street, a little freckle-faced boy of nine ran out into the street and shouted at him in a voice as thin and squeaking as the sound of a finger nail on a pane of glass. It was little Dave Trainor, "Chuck" Trainor's boy. Some of the neighboring women heard and saw what followed.