I agree the Norks probably weren’t responsible. While the Sony movie lampooned Kim Jung-un, who in North Korea would ever see it? If the Norks had the capability to do such a hack why just use it to release salacious Sony email? Wouldn’t the US financial system or electrical grid be better targets? It would be a good bet that it was an inside job by a disgruntled Sony employee who conveniently used this movie as a cover and North Korea as a patsy.
That's why I think it was an inside job. And FWIW, the so-called "hacker group", "Guardians Of Peace" doesn't exist.