"Chaos had advanced. The kitchen was a shambles of broken glass and china. The dining-room was stripped of parquet, the skirting was up, the door had been taken off its hinges, and the destroyers had moved up a floor. Streaks of light came in through the closed shutters where they worked with the seriousness of creators - and destruction after all is a form of creation. A kind of imagination had seen this house as it had now become."
The Second World War had just finished and the boys were living in a hostile and indifferent world. They had had an idea, a dream, and using their imagination the gang had created a plan (p.176 "a wild invention") to destroy Old Misery's house. To accomplish their mission they needed energy, hard work and organisation, just like in a war. However, they weren't doing it for hate, because hate requires a certain degree of passion and each and every one of them was cold-minded, and just followed the orders of their leader: Trevor, who was a mastermind of evil.
It's clear that the gang wanted prestige, distinction and fame (p.174 "The fame [...] been destroyed"); nevertheless it wasn't just for that; they were doing it naturally and consciously and most significant: for fun. Destruction was part of them, maybe because they were "the sons of war" and they were living a period of transition when many important changes take place; but as far as I'm concerned, proportional episodes (with proportional consequences) happen nowadays. This leads me to think that "destruction" is part of human nature.
Regarding their action, when the house was being smashed, sounds of carpentry were heard (creation) (e.g.: a clickety-clack, a bang bang, a scraping, a creaking ...). The boys created destruction and they had to...