Why Do People Do Evil Things?

The short story I read was "The Destructors" by Graham Greene.  I was amazed at the kids absolute evil.  There was no purpose to their evilness, except the fact that they wanted to cause destruction and unhappiness.  In this case the definition of evil would have to be "morally wrong or bad; immoral; wicked; harmful).  The author made a point of connecting power with evil.  For example, when T told the gang his plan to destroy Mr.Tomas' home, he took control over the gang and there he had the power.  He took this power from the original "leader" Blackie (who wasn't too thrilled about it).  To continue, he used his power to make "his" gang commit an evil deed.