What is the importance in the ending of "The Destructors"?
You could argue that the answer to this question relates to the class system in Britain and the impact that the war has had on it. Note the way that in the last paragraph, the driver remembers the house that "stood there with such dignity between the bomb sites like a man in a top hat." In Britain, top hats were worn by those of the upper class, the privileged few, and thus we can take it as a symbol representing the wealthy and dignified, and the class that had previously occupied a position of supremacy.
By his laughter and his reaction, the author of this excellent short story is thus suggesting that the impact of the war and the alienation is has caused has weakened, if not destroyed, the respect and deference that was once owed to the ruling classes.