How does priestley create such a striking end to the play "An Inspector Calls"?

The interrogation of Eric, which begins this act, is the last in a chain of interrogations which have structured the play since the Inspector’s arrival (in order: Birling, Sheila, Gerald, Mrs. Birling, Eric). Each of the Birlings has played a part in Eva Smith’s death, and each of them must take part of the responsibility for what happened to her and for her final, sad choice. 

Priestley makes a fascinating psychological point regarding the ways people react to guilt and responsibility in this last act. The heady, breathless glee with which Mr. and Mrs. Birling react is incredibly well-observed. As more and more pieces of evidence fall into place, Birling, in particular, is so overjoyed and relieved that he even dares to imitate the Inspector’s final speech. The point, clearly, is that some people are always unwilling to accept responsibility, no matter how clearly it is explained to them.

The sociological point is this unusually portentous. If man will not learn to look out for his fellow man in small ways, Priestley seems to argue, then man will destroy man on battlefields, with bombs, with guns, with “fire and blood and anguish.”