If we were talking about any country that wasn't the DPRK, I would agree one hundred percent, because it's absurd.
That said, the normal rules don't apply to the DPRK, because by normal standards, the DPRK _is_ absurd. The entire country is built around a cult of personality centered on the Kim family, and propped up by the military elites. While it's dubious to what degree people actually buy in to it, it's very clear that they have to pretend to do so, because the alternative is pretty much risking getting sent to a prison camp for the rest of your life, along with your entire extended family.
I find it entirely plausible as a motive (nevermind Sony being a Japanese company, whom they have no particular love for). That's not to say it's proof by any means, and there are certainly other plausible explanations - but I don't think it's fair to rule out the possibility on grounds that "this isn't what normal nations do."
As for "could", they've pretty much also been accused of all sorts of hacking activity against South Korea. I guess if you don't believe they did that, then sure - but they're pretty much the number one suspect.
And lastly, the notion that Sony Pictures would go to this length to promote a movie, to the point of wrecking their operations and embarrassing the absolute crap out of their senior executives, getting lots of people fired... it's insane. They're going to spend more just fighting off the lawsuits alone, nevermind the cleanup, than they will ever make on "The Interview."