After understanding the danger Samuel would become, he decides to burn the film and return to India, but Samuel finds him and tries to make him speak, finally killing him in his motel room. Hiro fixes Samuel's mistake by saving the film, but also saves Mohinder's life, thus changing the timeline.
Samuel's actions lead to a dangerous situation that requires intervention.  Hiro's actions have significant consequences for both Samuel and Mohinder, altering the timeline.