What makes fast decompression so dangerous for the fuselage? There have been flight accidents which involved structural failure after fast decompression of the cabin, like
BOAC Flight 781.

I'm  thinking about how decompression can destabilize or destroy the structure of a fuselage;

I would think that a hole pinched into the cabin wall of 1 cm diameter will just cause slow decompression over minutes.

I got the impression that creating a larger hole, like removing a door, can potentially cause severe failure of the fuselage in other places than the hole.  

If that is the case: what makes a hole large enough to be dangerous?
And what is the mechanism making a large hole dangerous?

Or is it that the structural failure comes first, and decompression is just a salient symptom of it?
Often it's the structural failure that results in decompression.

The comet (the aircraft in your example) was vulnerable to fatigue cracking that would propagate throughout the fuselage and compromise the structural integrity. Nowadays the fuselage is designed to stop such cracks from reaching further than the next spar and planes are retired when the number of cycles has reached the design limit.

There are factors to take into account when the vessel decompresses. If portions of the vessel are closed off from each other, then the pressure difference between those sections can exceed the tolerance of the divider and cause damage. This caused the floor to collapse when the cargo door blew off in Turkish Airlines Flight 981.