"The Lottery" begins without a clearly defined protagonist and focuses on the seemingly quaint traditions of the small town instead of one central character. However, as the story culminates, Tessie Hutchinson emerges as an unlikely protagonist as she cheerfully shows up to the lottery proceedings late. As the lottery drawing reveals Tessie as the unlucky recipient of the black spot, the young wife and mother protests the development by crying out "It isn't fair." The antagonist of "The Lottery" becomes the townspeople themselves, as they turn against Tessie with stones in hand, overwhelmingly supporting the continued use of the lottery and the brutal custom of stoning the lottery winner.
As other commenters have noted, Tessie Hutchinson emerges as the protagonist of the story by the end, but the protagonist really could have been any one of the townspeople. They are all, even Tessie, complicit with the lottery initially, and it isn't until Tessie pulls the marked paper that she objects to the tradition. However nervous people feel, and despite the fact that other towns have done away with the practice, no one says anything about giving up the lottery here. It's as though everyone is willing to take their chances because they know that odds are in their favor; there is such a small chance of being chosen. Therefore, no one speaks up, before the lottery, to point out the unfairness of the practice whose origins they cannot even recall, Tessie included.
Likewise, the antagonist is society at large, but it is important to note that Tessie would have been part of this group had it not been her who was chosen. She was all too happy to conform, making jokes about leaving her dishes and bossing her husband, until her number comes up. The conflict is revealed to be one of character (Tessie) vs. society (everyone but Tessie) as a result of the fact that no one speaks up about the injustice of the lottery until it is too late for that one person who now begs everyone else to join her.
Great question. My students always balk at this question after reading this story for the first time. At first the protagonist might seem like the townspeople themselves, with no obvious antagonist at the start. That's because the town and the people seem so peaceful and happy. The children are running around playing, and the adults are gathering together for some kind of social event. Everything seems wonderful and relaxed. But by the end of the story, the clear protagonist is Tessie Hutchinson. The reader, like Tessie, feels that the entire situation is cruel, unfair, and antiquated. The reader feels a large amount of anger toward the townspeople, and for that reason, the town, its residents, and its rules becomes the antagonist of the story. If I had to pick a specific person to be the antagonist, I would choose Old Man Warner. He seems 100% in favor of keeping the lottery. At one point in the story, Mrs. Adams mentions that some towns have quit the lottery completely. Old Man Warner replies, "Nothing but trouble in that. Pack of young fools."