Nina (Natalie Portman) performs an awful first act. She returns to her dressing room and engages and subsequently stabs who she thinks is Lilly (Mila Kunis), but is actually herself. (Nina returned to her dressing room, she saw Lilly there getting dressed for the black swan routine. They get into a fight and she stabs and kills Lilly. However, Lilly was never there to begin with, she was simply stabbing herself)
She performs the second act, the black swan routine, perfectly. She performs the third act perfectly and then dies from the stab wound.
Her obsession with being perfect causes her to reenact the story of Swan Lake itself.