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The main thing I see, in such a novel, is avoiding having this insanity be the equivalent of a deus ex machina that ultimately defeats him. I think the hero must defeat him in a way that would work...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/31558 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/31558 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
The main thing I see, in such a novel, is avoiding having this insanity be the equivalent of a deus ex machina that ultimately defeats him. I think the hero must defeat him in a way that would work **even if he were not insane.** It would be an unsatisfying ending if after a long campaign in which Dr. Nutjob came close to defeating the hero several times, but finally gets distracted by something shiny so the hero can put a bullet in his head. If your villain is insane but a daunting adversary throughout the story, he must be insane and a daunting adversary to the very end: His insanity can make him terrifying and unpredictable and thoroughly hated, but it cannot be the source of his defeat.