By being uneducated, Fantine is naive. There is much she does not understand about the business world, and she is often confused by the intellectual jesting speech of people around her. She confusing jokes with sincere emotions, which leads her into her catastrophic relationship with Tholomyes. She trusts him, she trusts the Thenardiers, she trusts the integrity of her co-workers, and in all of that she is disastrously mistaken.

Furthermore, her inability to secure better employment because she lacks the skills attending an educated citizen is a major contributor to her eventual descent into prostitution, and thus sickness and death.

Hugo wanted to portray the travesties brought down upon the poor and uneducated by an unforgiving society, but by contrasting Fantine with the Thenardiers, he is able to show that poverty does not necessarily lead to moral corruption (Fantine's prostitution notwithstanding).

How does Fantine contribute to her own demise?

The answer to how Fantine contributes to her own demise is an emotionally charged one in contemporary Western society. The answer is that she contributed through the act of intercourse with a man she was not married to. In Hugo’s, this action (by whatever means, love, seduction, molestation, voluntary engagement, etc.) reduces a woman to the categorization of "indecent," a categorization which can--and, for Fantine, did--prevent a woman from having or keeping employment. For Fantaine, once her coworkers found out about her baby Cosette who was born out of wedlock they successfully agitated to have her fired as being indecent. This reduced Fantine to the lowest means possible to earn money for food and shelter--selling her hair, teeth and finally her very body. Society then treated this final degradation as immaterial because Fantine was already indecent, the final lowering of herself to prostitution was, in that society's eyes, a repetition of what had already been. It is important to note that part of the reason Hugo wrote Les Miserable was to protest the very situation that Fantine lives through. He believed that the restraints of society described above were wrong and desperately in need of reform.