This sort of character departure is known by a common phrase that most people misunderstand because it was mistranslated for them: 'tragic flaw'. The exact word in ancient greek is 'hamartia', and its literal meaning is something more along the lines of 'misapprehension' or 'misjudgement'. The hero (or villain) experiences a moment in which they let their emotions get the better of them. This tragic instant of miscalculation then sets in motion the events that lead to their doom (death or misfortune, usually).

Oedipus, for example, gets tremendously angry when he gets into an argument with another man, and ends up killing him (the man turns out to be his father). You might say, until that moment in the story Oedipus has shown no signs of extraordinary temper, why would he suddenly lose his cool like that? Well, that's what hamartia is all about. It isn't that the hero (or just character) has a deeply ingrained characteristic that explains all of his choices throughout the story. There would be no human interest in that. Our lives are exceedingly messy. People make bad choices all of the time. Instead, nearly perfect men (like Oedipus or Achilles or Heracles) are humanized by making the sorts of mistakes everybody makes. Their greatness doesn't protect them from error, it just makes the consequences of their errors catastrophic.

Achilles gets so angry at the death of Patroclus that he disrespects the body of Hector and denies Priam his right as father to bury his son. In Ionic Greek the first word of the Iliad is 'menin', wrath, to tell the reader that the story is really about Achilles' moment of tragic misjudgment: when he succumbs to overwhelming rage and disrespects the gods and the rules of civilization. Achilles isn't a terribly wrathful person until that point; in fact, he's a little whiny. It's not bad characterization that his temper isn't displayed earlier, it's just known to the audience that every man has his triggers. It just so happens that because Achilles is the son of a goddess, when he gets mad he doesn't just get angry, he experiences 'menin': godlike wrath that is all-consuming and unstoppable.

Fast forward two thousand years and Faust, in his pride, misunderstands Mephistopheles' warning about the consequences of sin. Faust is otherwise a learned and wise man. For him to misapprehend in such a way seems like a departure from his character. It isn't; his emotions cause him to misjudge. Emotionality is usually the trigger for moments of hamartia. The Greeks were big on self control.

And so on. Even people who don't really understand the original concept of hamartia tend to get the story elements correct. We imitate early stories so closely that it's pretty hard to get it wrong.

So when things like that occur in a story, I try to give authors the benefit of the doubt. Sometimes [often] it's a result of bad writing, but other times I tend to think it's just authors following in the grand tradition of western literature. We attempt to humanize our heroes by demonstrating their moments of humanity (i.e. fallibility). Unfortunately, readers have come to expect their stories to be more like morality plays (another literary tradition), in which characters are very consistent and not terribly prone to human error.

Just my take on things. Not worth more than 2 cents.