The story suggests that destruction is part of human nature. It is infinitely more easy to destroy something beautiful, than it is to create it. The boys in the Wormsley Common gang are a microcosm of the senseless destruction and total war ideology of World War II.
I do agree with the posts above, however I also believe that this story is making a wider point about the impact of war on human nature and the resulting destruction - both physical and moral. Remember that Greene chose to set this story in the aftermath of World War II in a bomb-torn London which had seen so much destruction and death. Out of the ashes of this conflict a whole generation of children had grown up who knew nothing except the realities of war. This finds its fullest expression in the character of T. who is completely nihilistic and detached from emotion. Greene seems to be painting a grim picture of the effects of war on the human nature and how it can divorce us from our moral and emotional anchors.
The story is ultimately about power and not simply the power struggles between the boys in the gang, but allegorically about the power struggles between Sri Lanka maintaining its independent status and it being destroyed from the inside out and completely changed into something different. The house allegorically represented the post independent Sri Lankan state and the power struggle between the boys actually represents the power struggle between Blackie (a.k.a. UNP of Senanayakes, Jayewardene and Wickremasinghe (all belonging to the same extended family) ) and Trevor or T. (a.k.a. SLFP of Bandaranaikes (Father, Mother, Daughter and Son – a nuclear family) ). Mike represents the military. Each part of the story represents the events that took place to destroy the post independent Sri Lankan state. So we can see that while the story seems to have two levels, one of action and suspense about boys destroying a home, and another about human nature, we see that this story actually operates on several levels when we see its allegorical effects as well.
A central theme to this story is power. The boys struggle with the shifting power in their little community, and Mr. Thomas struggles with his mistaken belief that he still has power. Mr. Thomas assumes he does because he is an elder, and they are still boys. However, the boys have learned at an early age how to gain their own power and how important power is. Blackie gives in to the power of T. when the boys give their allegiance to him. Blackie knows he must accept T.'s power because that is the way of things - submit to power or be tossed into open war.
This theme suggests that human nature is geared towards domination. Humans want to have power - the history of human development is a timeline of communities gaining power over each other. If humans can't have power, they will respect another's power. If that leader wavers, then the power shifts and the followers will pick a better leader.