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We tend to think of fictional characters in terms of "Hero" and "Villain" when this should not be a case. The character of focus in your story is the "Protagonist" and the element directly in his ...
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Source: https://writers.stackexchange.com/a/48649 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#2: Initial revision
We tend to think of fictional characters in terms of "Hero" and "Villain" when this should not be a case. The character of focus in your story is the "Protagonist" and the element directly in his or her path from achieving his or her goal is the "Antagonist". These terms do not mean good and evil binaries but rather their relation both to the audience and the other character. Who's side is the story showing to the audience. A protagonist need not be good and an antagonist need not be evil. Consider "Breaking Bad" where the meth cooking Walt is a protagonist to his DEA Brother-in-law who's horror at the depravities of the drug kingpins in his jurisdiction resolves him more to stop the biggest Kingpin, even if he's family. Similarly I make the argument that the antagonist in Mulan isn't Shan Yu (the name for the boring Hun leader, who is clearly a villain). Shan Yu really doesn't cause any problems for Mulan's victory and has nothing against her. It's Mulan's society and their strict enforcement of gender roles that keep her from kicking Shan Yu's ass. Compare the scene where Mulan's CO, Captain Shang, learns Mulan is a woman vs. Shan Yu's discovery that Mulan is "The Soldier from the Mountain." While both nearly kill her, the former is because her culture holds that any woman who joins the army (a boys only club) should be killed, where as "The Soldier from the Mountain" is the one person who handed Shan Yu a massive defeat despite Yu's superior numbers, and thus the biggest threat in the room. Just because the killer in your story has a backstory you feel is worth telling, doesn't mean he's being protrayed as a hero for his actions, but that he is being portrayed as a protaganist in a perspective flip on the story we first saw. This is hardly new, as Malificent, Wicked, Paradise Lost, and arguably MacBeth are told from what should be the point of view of what would be terrible people who did terrible things for reasons. The recent film Avengers: Infinity War gave us great sympathy for a purple cgi giant... who mass murdered half of all life in the universe. Untold numbers of sentient creatures were wiped out by this genocidal monster and the film was hyped as "his story" for all intents and purposes. And what makes him so scary... is that the film shows he's capable of very powerful love and he's not seeking power or glory or accolades for his mass murder. He feels like everyone knows the answer to the problem but no one is willing to live with doing the job... so he'll bear that guilt for the greater good.