The theme of "A Piece of String" has to do with the meaness, cruelty, and injustice of humanity. Maupassant often wrote stories about human selfishness, wickedness, envy, spite, greed, and other bad qualities. Here are a few pertinent quotations:
"Everyone is perfidious, a liar and a phony. Everyone wears a false face."---Guy de Maupassant
Maupassant persuades us to accept his illusion that cunning, ferocity, greed, and coarseness are more common among men than we hope they are.---Wallace Stegner
The thing that most tormented De Maupassant, to which he returns many times, is the painful state of loneliness, spiritual loneliness, of man, of that bar which stands between man and his fellows; a bar which, as he says, is the more painfully felt, the nearer the bodily connection.---Leo Tolstoy
Maupassant was deeply influenced by the pessimistic German philosopher Artur Schopenhauer, who had an even lower opinion of human nature. In "A Piece of String" an entire village makes a simple, humble man's life miserable by accusing him of theft. There is nothinig he can do to convince them that he was only picking up a piece of striing. His accusers don't want to believe him because they enjoy venting their spite on a defenseless man.
"A Piece of String" might be compared with Shirley Jackson's story "The Lottery" and with Maupassant's famous story "Boule de Suif."
Maupassant's story is about pettiness and hypocrisy. None of the characters, including the protagonist Hauchecome, come off as virtuous. It’s Hauchecome’s extreme greed which makes it impossible for him to pass up the piece of string on the ground; his stooping to pick it up, while entirely consistent with his skinflint values (for which he is famous in the village), nevertheless provides the opening for his enemy Malandain to accuse him of the theft of the wallet. Hauchecome is brought before the mayor, and, despite protestations of innocence, everyone in the village believes that he is guilty, a belief that continues even after the wallet is found. It does not seem to matter if he did it or not; all that counts is his vulnerability to attack, something that everyone exploits to the best of their ability. Hauchecome becomes an outcast and a laughingstock.
On the face of it, you could say that the theme of the story is man’s inhumanity to man, or perhaps how people become like animals and will turn on companions when they become too weak to defend themselves. There does not seem to be much honor in the story, and yet Hauchecome is fixated on trying to repair his reputation—in itself a kind of hypocrisy, since he was never truer to himself than when he stooped to pick up the string. In fact, though they all behave badly, the villagers are true to their core values—it’s just that those values are ruthlessness, greed, suspicion, and hatred. So perhaps the theme is about the mutability of truth. Even though Hauchecome did not steal the wallet, he has a “sharp” reputation which makes everyone believe he did it.