It may very well be an employee or a group of employees who did this with no outside influence. If so they were very foolish to think this wouldn't get traced back to them.
Why would such a group have a motive to do so much damage to Sony and attract the interest of the FBI and counter terrorism? Again, if they were not motivated by outside forces this was a very foolish thing for them to do.
Now one of the things I have long done for a living is repair electronic equipment. When I encounter a problem with a piece of equipment I consider the symptoms and then I ponder what component failures could cause such symptoms. I find that often there are many suspects that could produce the exact same symptoms so I find myself having to consider multiple plausible theories at the same time.
I have to devise tests to support or disprove the theories until I narrow it down to one theory which fits both the original observed symptoms and all my subsequent tests. Using this technique I eventually narrow it down to a single component failure.
My point here is that It is normal and natural for me to consider multiple possibilities until further information is forthcoming.
Yes, it *COULD* be North Korea using their human assets and money to make this happen. They had the best motive of anyone so far seen in this drama.
Yes, it *COULD* be disgruntled employees acting alone and with no connection with North Korea, though if this is a case, they badly overestimated their skills at getting away with it.
Yes, it *COULD* be a third party of which we are currently unaware.
Again, I'm perfectly content with juggling multiple theories and weighing probabilities until more facts come forward to narrow down the options. This is how I have been solving problems for quite a long time.