See the text of the dialogue between Bill and Kiddo below. The situation at the time was that Bill injured Kiddo with a poison dart, and then slowly began to talk about his theory
about superhero : there are a series of comics about superhero in the United States, such as Batman and Spiderman. Among them, only Superman is different from others. Every superhero is divided into the id and the superego. Like Spider-Man, I am Peter Parker. Parker is a normal person. He needs to put on the coat of Spider-Man to become a superhero with super powers.
And Superman is unique in this point. Superman was born Superman. He doesn't need any extra help (such as props, costumes) to help him become a superman. On the contrary, after putting on ordinary clothes, Superman pretends to be an ordinary person, a cowardly and timid ordinary person. Superman's id is a superhero, but his "superego" is a person with all kinds of defects, like you and me.
Kiddo was pregnant (being Bill's child) and decided to wash his hands and lead a normal life. She went to a small town to get married. And Bill followed and killed Kiddo's fiancé and his family.
What Bill wants to tell Kiddo is that Kiddo's self is a killer, and this is something she cannot change. She can try the lives of ordinary people incognito, but in her heart, the nature of the killer will not change.
What Bill didn't say, and the movie didn't give a conclusion is whether Kiddo would be happy if she really started a new life incognito, an ordinary person, and a husband and a child. Bill's meaning seems to be negative. I understand that this is also the motivation for Bill to kill her fiance-not because of Kiddo's betrayal. Bill seems to have the meaning of walking for the sky. Or, Bill is actually trying to save Kiddo from a life she is not happy about, as if I had (unsuccessfully) persuaded my friends not to work in investment banking.
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[The dialogue between Bill and The Bride/Kiddo before the duel. From imdb]
Bill: As you know, l'm quite keen on comic books. Especially the ones about superheroes. I find the whole mythology surrounding superheroes fascinating. Take my favorite superhero, Superman. Not a great comic book. Not particularly well-drawn. But the mythology... The mythology is not only great, it's unique.
The Bride: [who still has a needle in her leg] How long does this shit take to go into effect?
Bill: About two minutes, just long enough for me to finish my point. Now, a staple of the superhero mythology is, there's the superhero and there's the alter ego. Batman is actually Bruce Wayne, Spider-Man is actually Peter Parker. When that character wakes up in the morning, he's Peter Parker. He has to put on a costume to become Spider-Man. And it is in that characteristic Superman stands alone. Superman didn't become Superman. Superman was born Superman. When Superman wakes up in the morning, he's Superman. His alter ego is Clark Kent. His outfit with the big red "S", that's the blanket he was wrapped in as a baby when the Kents found him. Those are his clothes. What Kent wears- the glasses, the business suit-that's the costume. That's the costume Superman wears to blend in with us.Clark Kent is how Superman views us. And what are the characteristics of Clark Kent. He's weak... he's unsure of himself... he's a coward. Clark Kent is Superman's critique on the whole human race. Sorta like Beatrix Kiddo and Mrs . Tommy Plimpton.
The Bride: Aso. The point emerges.
Bill: You would've worn the costume of Arlene Plimpton. But you were born Beatrix Kiddo. And every morning when you woke up, you'd still be Beatrix Kiddo. Oh, you can take the needle out.
The Bride: [does so] Are you calling me a superhero?
Bill: I'm calling you a killer. A natural born killer. You always have been, and you always will be. Moving to El Paso, working in a used record store, goin' to the movies with Tommy, clipping coupons. That's you, trying to disguise yourself as a worker bee That's you tryin' to blend in with the hive. But you're not a worker bee. You' re a renegade killer bee. And no matter how much beer you drank or barbecue you ate or how fat your ass got, nothing in the world would ever change that.
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