I think that Norrington in DMC was a person figuratively at the end of his rope. There are essentially two Norringtons in his character - the stiff-upper-lip British officer for whom King, Country and Duty are paramount even above life itself, and the man James Norrington who is smarter than many people make him out to be (they don't give commodore commands to dummies!) and who knows the right thing to do when the time comes - remember that speech that Keira gives Jack (Jack replies: "I like those moments... I love to wave at them as they pass by!") - Norrington has those moments too and he doesn't just 'wave at them' but on occasion he seizes them for the greater good. Hence his allowing Jack Sparrow a day's head start, hence his acceding to Will and Elizabeth's marriage even above his own heart's desire.
Now Norrington in DMC is a man who has just about lost all that. He's lost the Dauntless - no small thing, considering an 80 gun ship of the line would have something like a thousand PLUS men manning her. Care to have a thousand or so souls on your conscience? Not to mention he failed to capture Sparrow, lost his commission and was a wanted man. Plus his beloved is set to marry the once bedraggled wretch that HE HIMSELF SAVED in Curse of the Black Pearl. Norrington SAVED Will Turner at the very start of the series. And Will Turner takes everything (it seems) away from him, prodigiously aided by Jack Sparrow of course.
Norrington by an ironic twist is forced to explore his 'dark side', he has to crawl out of this hell and find 'redemption' as he aptly puts it even if he has to become everything he despised as a British officer = He becomes a drunken, light-fingered, sneaky pirate!
All to recover his lost honour, which by the way, IS a VERY important thing. Honour (at least it used to be) even as important as life or an afterlife. For the ancient Egyptian, the glory and honour one won in life was reflected in monuments and hieroglyphs and were necessary to obtain a place in the afterlife - if you didn't have any your heart was chucked out to a great heart-eating monster and you just simply ceased to exist. For the ancient Greek of the temper of Achilles, honour or 'kleos' was tantamount to immortality as well as it was what people say about you (when you are gone or are dead).
http://en.wikipedia.org/wiki/Kleos (Trivia: The legendary Cleopatra's name means literally 'the Glory' [kleos] 'of her Father' [patrae])
And from these concepts, our own concepts of honour developed. Honour for Norrington was the over-arching consideration. Something for which everything even personal dignity was to be sacrificed.
But by AWE he realizes that in his quest for honour he has done irrevocable harm to those he loves and raised to power a man most dangerous and implacable. So he 'choses a side'.
Anyways, that's the way I see James Norrington from DMC to AWE. Plus he looks rogue-ishly cool as Scruffington - and he has a pretty mean swordfight to boot!