And who's to say that adding another villain would add nothing to the plot? It all depends on how well it's executed. I feel like storytelling rules like "kiss" are a hindrance. The rules of telling a story shouldn't be strict rules. They should be guidelines, subject to being ignored or revised based upon circumstances. And I can think of two perfectly good examples of super hero movies where they had multiple villains and it worked. Those being Christopher Nolan's Batman films. Let's count the number of Batman villains present in the films so far. In Begins, you had Ra's Al Ghul, The Scarecrow, Carmine Falcone, Detective Flass, Joe Chill, Mr. Zsasz, and two characters who essentially filled the role of Ra's Al Ghul's bodyguard Ubu. In The Dark Knight, you have The Joker, Two Face, Sal Maroni, Scarecrow, and various mobsters. It's all about striking the right balance between the characters and the plot. Saying that having more than one villain eats up screen time that is needed elsewhere can only be a valid statement if you actually know what the plot is. Otherwise, having that other character could work with the plot beautifully.
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