Who is the antagonist and protagonist in "A Piece of String" by Guy de Maupassant?

In this story, I think it is quite clear that the
protagonist is Maitre Hauchecorne.  He is the one that we sympathize with in the story
and he is the one we are really concerned with -- hoping things come out okay for
him.

I think you can say that society (and specifically the
peasants in general) is the antagonist.  It is society that refuses to believe Maitre
Hauchecorne and that eventually causes him to go crazy.  The author thought peasants
were too suspicious of one another and he is trying to show that by having peasants in
general be the antagonist in this story.