Accepting that the "sleeper fanatic" scenario is unlikely, how about a cabin crew member who is just a disgruntled employee? Somebody who is overworked, jet-lagged, poorly paid and feels unappreciated by the company? Possibly with added stress such as boyfriend/girlfriend trouble, being in a load of debt and away from the support network of family and friends. This is not just a Middle East scenario, it could happen in a lot of airlines in the world.
Some airlines have tens of thousands of cabin crew. It is simple statistics that a few of those are going to be depressed and some of those will be suicidal. Thanks to the publicity of recent days they now know it is possible to bring down an aircraft with a simple turn of a knob in less than 10 minutes. And they know where the crash axe is.
It seems to me that the two crew in the cockpit rule is a knee jerk reaction which potentially could cause as many problems as it solves.