I think you could argue that the one of the primary antagonists of the story is the young man who died on the day of the party. It is his death that ultimately has the most profound affect on Laura and her understanding of life and death.
Before the news of the young man's death, the day is a beautiful picture of loveliness. Laura is eating bread and butter, pretending to have more power than she really has as she orders the workers around, and engaging in rather silly chatter with her mother and siblings. Once she hears of the accident and the man's death, she immediately thinks the party should be cancelled. This reaction is perfect proof of her youth and naivete. She thinks that the life events of the lower classes should affect her, and her mother scoffs at the very thought of that idea -- essentially questioning why on earth Laura would see it that way. The upper classes don't change their plans because of the life events of the lower classes -- especially in regards to people they don't even know! She convinces Laura that they really must go ahead with the plans, and she even uses Laura's grand new hat as the enticement to get her daughter to drop her foolish notions. It works because at this point in the story, Laura is still young and naive.
But the thing that makes this story great is the fact that Laura just can't let it go. Once the party is over she is right back to thinking about the dead man. She insists that some of the party leftovers should be packed up for the dead man's family, and she ends up being the one to bring them down to them. It is her mother who ends up being an antagonist here because she is a catalyst to these decisions. Once Laura is walking down the lane and she arrives at the man's house, she feels the distinct difference between herself and these people. She feels out of place, especially because of her fancy hat. She is at a loss for words. But the most important thing that happens in the story happens as she faces the dead man. She notices that the man has a contented look on his face, and she thinks to herself that he has come to a place of peace, for by his death he is released from the pain and drudgery of human existence. He is freed from all the troubles of life. Laura understands that death is not a gross event or something to be feared, but that it is a natural part of life. When she tries to express this idea to her brother she literally without words, but the reader understands how profoundly different she is for having gone to this man's house and having this experience.