I think that part of the significance of the ending is that it helps to bring out the reality of the world following the wanton destruction that was the child of World War II.  The effect of the bombing on London is evident.  There is nothing but destruction that surrounds the people of London.  Being exposed to this on a consistent basis makes such destruction a part of their being.  There is a lack of emotional affect in such a condition, seeing destruction as a daily part of existence.  This is why the boys are able to contemplate the destruction of the house in a such an easy manner, being able to do so without much in way of emotional reflection.  Greene seems to be suggesting that it might be easy to lay this solely at the foot of the youth.  Yet, the reaction of the drive to Mr. Thomas' "sobbing cry" reflects how empty so much of society has become as a result of the war.  When the driver recalls the destruction of the house, he is only able to laugh at its destruction.  The ending effect of this is a world devoid of personal connection.  Consider the closing words of the story:  "I'm sorry.  I can't help it, Mr. Thomas.  There's nothing personal, but you got to admit it's funny."  This line reflects how society has been constructed as a result of the bombing as "nothing personal."  It is this ending in which Greene has been able to develop a thematic element in which one sees how the world has changed, shifted from a setting where emotional attachments were either valued or understood to one in which there is "nothing personal."  In this void of personal connection, destruction becomes easy and something to which laughter and derisive division is morally acceptable.

Graham Greene gives an unusual ending to "The Destructors." Discuss in detail the effects of such an ending. 

There is an emotional isolation that is present at the end of the story. The conclusion that Greene renders is one in which an emotional disconnect is evident everywhere.  Mr. Thomas has been locked in a bathroom.  He has not been brutalized, but he has been locked in a bathroom by the kids.  Mr. Thomas' house has been torn down to complete rubble and the reaction from the truck driver is to laugh at its gutted remains.  When Mr. Thomas admonishes him, the driver responds with that it is "nothing personal" and that the situation is "funny" to him.  The ending of Greene's story creates a world of emotional divergence.  No one really seems to understand anyone.  The kids don't understand the emotional impact of what they have done.  Mr. Thomas doesn't understand why no one seems to have an emotional response to what they are seeing.  The truck driver finds the destruction of one's hopes and property as completely filled with humor.  When the truck driver utters the words, "It's nothing personal," these words seem to summarize the emotional climate established at the end of the short story.  The real destructors seem to be all of the inhabitants where an emotional affect cannot be grasped or understood.  It is in this ending where the story is both unusual and thought provoking.