Everything about the attack has seemed to be inconsistent with North Korea's tendency towards propaganda.
It just seems... odd... that the attackers behaved consistently like disgruntled employees/ex-employees.
Then Sony started talking about North Korea for whatever reason, and I think the attackers saw that and ran with it, thinking it was a great way to send Sony on a wilde goose chase. Heck, they might have intentionally left evidence pointing towards North Korea from the beginning (I suspect the various tools that the FBI thinks imply NK have already been traded around via underground methods and are in the possession of people other than their original creators/users...). Once there was public talk of NK, I think the attackers just decided it would be effective to screw with Sony regarding "The Interview". It's probably nothing to do with any moral objections to the movie - but it's a great way to cause a major financial loss for Sony and make them think someone else is responsible.