Monomaniac

To talk nonsense, or poetry, or the dash between the two, in a tone of profound sincerity, and to enunciate solemn discordances with received opinion so seriously as to convey the impression of a spiritual insight, is the peculiar gift by which monomaniacs, having first persuaded themselves, contrive to influence their neighbours, and through them to make conquest of a good half of the world, for good or for ill.

Harry knew now that the old man was crazy, or at least a monomaniac, and, though he seemed harmless enough, it was of course possible that he might be dangerous.

He had never yet been able to determine whether the old man was a consummate hypocrite or a religious monomaniac.

...then it was that monomaniac Ahab, furious with this tantalizing vicinity of his foe, which placed him all alive and helpless in the very jaws he hated; frenzied with all this, he seized the long bone with his naked hands, and wildly strove to wrench it from its gripe.