Derpy, the muffin mare of Ponyville (a tiny, pointless town on the backside of nowhere that nopony at all cares about) suddenly finds that she has fallen pregnant with her second daughter. With no means of supporting herself (let alone a foal), she begins trying to find an alternate source of non-muffin income.
Fortunately, Spoiled Rich offers her a position. A job killing ponies that displease her. And when it comes to Spoiled Rich, there are a LOT of ponies that displease her.
Derpy accepts the job, and performs about as well as would be expected.