Bad luck? Mathilde's pride or greed? Patriarchal society?
There's no "bad guy" in this story who opposes Mathilde and actively thwarts her desires at every turn. Mathilde's miserable at the beginning, but just what's responsible for making her miserable is debatable. It all depends on how you assign responsibility. You could say it's just her bad luck to be born into a middle-class family, or you could also think it's her own frustrated greed which makes her unhappy. You could even say that the antagonist is the patriarchal society in which she lives. After all, it's the man's world that forces her to live the middle class woman's life that bores her to death.
Likewise, who's responsible for Mathilde's fall into poverty? You could say it's the pride of Mathilde or her husband. If they'd only initially confessed everything to Mme. Forestier they could have avoided the whole mess. You could also say it's Mathilde's greed, which led her to borrow the necklace and made the situation possible in the first place. Then again, you could find the actions of Mathilde and her husband entirely understandable, and think it makes no sense to blame them. In that case, it's probably just a matter of bad, rotten luck.