"The Interview" is as provocative to North Korea as burning a Koran is to an Islamist

If the USA then punishes North Korea for this retaliation then there is a risk of a spiral of escalation whose outcome is unlikely to be happy. The North Korean suggestion of a joint investigation of the hacks is probably a recognition of this danger. It would at least buy North Korea some time, and it is likely that its hackers covered their traces sufficiently well that the FBI would not be able to prove North Korean involvement without revealing capabilities that it would prefer to keep secret. The FBI is unlikely to be enthusiastic about a joint investigation and many in Congress will be hostile to the idea too.

Meanwhile, Sony Pictures has announced that it plans after all to release “The Interview” on some other platform. North Korea cannot let this pass. It has made clear that it will do whatever it takes to stop the film and so is almost certain to attack either Sony Pictures again, or anyone else associated with the eventual launch, or both. If North Korea’s response this time is another large hack then the situation can probably be contained. But the hackers’ messages that caused cinemas to cancel screenings, and so forced Sony Pictures to cancel the launch, warned of a repeat of 9/11. This implies actual violence, perhaps against cinema-goers. It is unclear whether North Korea has the capacity to launch physical (rather than cyber-) terrorist attacks on American soil, but if it is so foolish as to do so then the US administration would be under immense pressure to respond firmly which could, again, lead to an unpredictable spiral of escalating moves and counter-moves.