I'm trying to remember the name of this short story. As far as I remember the guy wants someone killed so he hires a hitman to kill the person, but the hitman says he will get a discount if he chooses some more people to kill. The guy then finds some more people but the hitman says again that if he finds more X people the discount will be higher. This keeps repeating until the hitman offers to kill all of mankind for free (I think) and the guy chooses that. I remember that it ends with the guy hearing sounds and screams of people getting killed everywhere.
I think that was We can get them for you wholesale by Neil Gaiman.
The story is about Peter Pinter, a mild-mannered city-dweller who finds his fiancée unfaithful, and so, in the spirit of revenge, searches the phone book for an assassin. To his surprise, he finds just what he is looking for, and, to his curiosity, the company offers special deals and discounts for large orders. Peter happens to be a sucker for bargains, and soon finds himself in over his head.