Looking for a specific short story.....

[MfFly]

Several years ago I read a short story (from a magazine I am sure) about a city that was in panic over an unknown person that was executing people he came across in the day that were not polite. The part I remember most was the ending, where a convenience store clerk decided to be nice to everyone because nobody knew who the killer was and being rude got you killed.

Anyone know the name of this story or the author?