Based upon the many pictures here:
Russia Plane Crash Debris Found in Egypt's Sinai Desert - NBC News
I don't see any sign of an explosive decompression. Admittedly, the pictures are not complete but I think it not probable that a bomb or a missile brought this plane down.
I can't get a real sense of the debris field so I can't comment of the the mid-flight break-up. I will say that the structural forces shown in this limited sample of debris suggest a flatter landing. The fact the tail is relatively intact suggest either the nose hit first or the tail broke off in flight.
edit: I also will note that on picture four that break looks clean, which may support the structural failure of the tail theory. Any structural engineers care to opine on that issue?