Monomaniac

Karl is an egocentric monomaniac.

Source: tatoeba (10071808)

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.

Source: wiktionary

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.

Source: wiktionary

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

Source: wiktionary