How do Valjean, Javert, the supervisor, the Thenardiers, and Fantine contribute to Fantine's death?
I'm going to go in chronological order to show the plot development that leads to Fantine's death.
First, Fantine makes a poor decision to trust herself to the student Felix Tholomyes. She behaves rashly, and so has a daughter out of wedlock. This immediately makes a hard life for her. Then she chooses badly when picking the Thenardiers to care for her child. She picks too quickly and ends up with money hungry and devious people, leading to more difficulty.
Fantine's supervisor is too hard on her, though. She was making enough money, but when the supervisor fires her without considering other options, she dooms Fantine to unhealthy ways to get money.
Valjean should not have trusted the supervisor so much to handle the situation, though. He could have and would have helped if he had been paying attention.
By forcing Fantine to send more money than needed, the Thenardiers force her into an even more unhealthy lifestyle. Fantine becomes ill as a result.
Javert's treatment of Fantine helps to break her spirit, the only thing keeping her moving. Although Valjean intervenes at this point, it is too late. She tries to recover, but is unable to. Then, when Javert barges in to confront Valjean, her fear of being treated again as Javert did before robs her of what recovery she was making. She dies as a result of the conditions she has been put in by society, helped along by her own bad choices.