In "The Destructors," Graham Greene observes man's unfortunate tendency to choose destruction over preservation.

The setting Greene chooses, an English neighborhood that has endured profound destruction in recent bombings of the war, offers a backdrop that demonstrates the inevitable cycle of destruction and violence that mankind chooses. The decisions of adults to attempt to destroy each other is mirrored in the behavior of the boys. There is a struggle for dominance between Blackie and Trevor that can only be resolved with a winner and loser. There is no room in their world for voluntary collaboration. Mike is kept quiet with threats of violence against him: “If you don’t shut your mouth,” somebody says, “you’ll get a frog down it.”

Trevor's decision to destroy the house designed by noted architect Sir Christopher Wren has elements of class struggle attached to it, but the overall idea is that something valuable that deserves to be preserved is instead destroyed. Greene explores the idea that preserving human life and cultural treasures are not the choices that adult men naturally make, so it should not be a surprise that boys tend to emulate these same unfortunate tendencies.

We can state the central theme in a couple of ways, but I think the main idea of this story is this:  Extreme circumstances can drive people to act in ways that do not fit their usual characters.

The boys in the gang had lost nearly everything to the bombings of WWII. They had lost their homes or feared for them daily.  Their neighborhood is described as having "...suffered from the blast of the bomb and the side walls were supported on wooden struts. A smaller bomb and some incendiaries had fallen beyond, so that the house stuck up like a jagged tooth...." They had lost their childhood. They no longer felt safe in their world.

Thus, they were driven to petty mischief which is normal for most boys, but when they meet Trevor and agree to his destructive plan, they are truly acting out of character.  Trevor's desire to wreck Old Misery's home was driven by jealousy, and he was able to draw the others in.

It is doubtful that any of the kids would have joined in the destruction if they were not disillusioned by the adult world to begin with.  They were victims of destruction themselves, so their destructive acts, while not condoned, are at least understandable.

What is the theme of "The Destructors"?  

The young teens who form Blackie's gang in theory should be at the age where they are beginning the process of losing their innocence and naivety. However, it is clear from their speech and actions that they have already reached this stage of cynical world-weariness. They have become old before their time, and in a very real sense have not been allowed a "proper" childhood due to the rigours and stresses of war. This is a fact they cannot escape, for they are literally surrounded by the detritus of wartime, meeting in a car park which was the site of the last bomb of the first blitz and surrounded by bombed buildings.

Interestingly they try to impose an "order" on to their surroundings but it is obvious that this order is merely destruction - as they themselves have been "destroyed" by the war, now they are only able to mimic these acts of destruction on their environment to express their anger and loss.