3. Its clear from the hard-coded paths and passwords in the malware that whoever wrote it had extensive knowledge of Sonys internal architecture and access to key passwords. While its plausible that an attacker could have built up this knowledge over time and then used it to make the malware, Occams razor suggests the simpler explanation of an insider. It also fits with the pure revenge tact that this started out as.
4. Whoever did this is in it for revenge. The info and access they had could have easily been used to cash out, yet, instead, they are making every effort to burn Sony down. Just think what they could have done with passwords to all of Sonys financial accounts? With the competitive intelligence in their business documents? From simple theft, to the sale of intellectual property, or even extortion the attackers had many ways to become rich. Yet, instead, they chose to dump the data, rendering it useless. Likewise, I find it hard to believe that a Nation State which lives by propaganda would be so willing to just throw away such an unprecedented level of access to the beating heart of Hollywood itself.
http://marcrogers.org/2014/12/18/why-the-sony-hack-is-unlikely-to-be-the-work-of-north-korea/
On an episode Vice detailing the struggles of North Korean refugees in South Korea, they claim they are recognized but the type of Korean they speak so one claimed he is from a certain area of South Korea which is very similar to the type in North Korea to avoid the bigotry.