It's also fun when sometimes the Hero is shown in a different light, and actually appears to be a complete dick. Take "Family of Blood". That's one of those rare times where you kind of really dislike The Doctor at the end, and you're kind of meant to. Doctor Who shades of Superdickery.

I'd say that in all likelihood it's because there IS a part of those characters within you. Chaos and Fawkes aren't simply moustache-twirling, one-dimensional villains; they're the versions of you without the instinct to Not Be A Dick. They are your honest dark side unleashed. It's why they're believable, and why they're so damn much fun. Sometimes you have to unleash The Dick Side, and it's preferable to do so as a fiction.