Doesn't take much to assassinate a few people. If they can hack Sony's back office database, they can locate people. NK managed to penetrate a closed society with no significant Korean community like Burma and almost take out the hardest of the hard targets - the South Korean president. They "failed" in that they only took out several South Korean cabinet members. Think of how McVeigh and Nichols managed to destroy the Murrah Federal Building. NK's operatives don't need to go after anything so heavily-protected. By comparison, Hollywood personalities are practically naked.
you are right about what Korea could do as far as violence. I still think that I am right about the info that has not been released