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This is the main plot point behind the Anime series, Maoyu. The "Hero" goes to fight the "Demon King" to end the war and suffering of all the human people, but ends up being turned to the Demon Kin...
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Source: https://writers.stackexchange.com/a/40626 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
This is the main plot point behind the Anime series, [Maoyu](https://crunchyroll.com/maoyu). The "Hero" goes to fight the "Demon King" to end the war and suffering of all the human people, but ends up being turned to the Demon King's side. The Anime accomplishes this in a few ways: - It doesn't give us much time at all to feel the Demon King is actually a terrible evil. - It makes the Demon King completely opposite of every expectation both the Hero and viewer have. - It takes pains to identify the "true evil" (and _show_ this time, instead of just tell) and dissociates the Demon King with it. These are the three biggest methods I see to defy the viewer's expectations and turn what might be hate into empathy. How you accomplish this is up to you, but your end goal shouldn't be convincing the Hero as much as convincing the the reader. If you can't win the reader over to the other side, they'll be stuck hating your main character, no matter how realistic it is for the MC to act this way.