And as the other reply alluded to, what would be the motive for anyone else besides North Korea? It would have to be a very psycho ex-employee to risk going to jail for the rest of their lives for no personal gain. The threat to bomb theaters showing the film doesn't fit the disgruntled employee theory at all.
And very targeted and embarrassing release of insider emails and documents doesn't really fit the North Korea theory very well. I mean, their *official spokesperson* released a statement (sic): "The U.S., a big country, started disturbing the Internet operation of major media of the DPRK, not knowing shame like children playing a tag." Just don't think they are going to be concerned that much with internal Hollywood politics when they can't even manage to translate one sentence into proper English.
Also, apparently the whole GoP reference and Interview theater threats only came up 3 weeks into the hack; one popular opinion is it was misdirection to muddy the investigation (if so, it sure worked!) And you'd think they'd lead with that if that was their original intent...
But anyway, at this point neither argument is very convincing. There just isn't any (public) hard evidence either way. Some claim the FBI has "proof they aren't showing" - if they want people to believe them, they might want to release that. The US government hasn't really built a very trusting relationship with its citizen these days...