These two genres go hand-in-hand (or should that be stone-on-head?) in "The Lottery."
By placing the story in a generic small town, the horror of "The Lottery's" ending stands in stark contrast to the normality of the story that comes before it. In fact, Jackson's portrayal of the small town fooled New Yorker readers so well that letters poured into the office demanding to know exactly which small town practiced the barbaric ritual of stoning.
The story's impact only increases upon multiple readings. Once you know the true purpose of the lottery, seemingly harmless details within the story take on gruesome dimensions. In the first paragraph, we learn that the villagers like to finish the lottery in time for lunch. This seems reasonable at first, but soon indicates the callousness with which the villagers treat a public execution.
Likewise, the boys' interest in stones can first be read as childish play, but takes on a sinister cast once we know the stones' purpose. These details demonstrate Jackson's true mastery of the horror genre.