Riza was dead the whole story. Roy was just hallucinating her. She had killed herself and he blamed himself and denied the idea that she killed herself. So he went crazy trying to find a killer that didn't exist. Riza never lets him say I love you, because he never got to tell her he did. So in the end he breaks into Riza's old apartment, now owned by a different woman, which is why he heard a woman scream, and grabbed a knife and killed himself thinking if only he had told her that he had loved her she wouldn't have killed herself, and he wanted to join her in the after life.
