What insights about human nature, human psychology, and human society does the The Lord of the Flies present? How are they expressed?

Yikes, I could write a giant paper on this. It's difficult to be brief, this novel is packed with this stuff. Again I'm going to refer you to the GradeSaver Themes page. Pay particular attention to "The Nature of Evil",

http://www.gradesaver.com/lord-of-the-flies/study-guide/major-themes/

http://www.gradesaver.com/lord-of-the-flies/study-guide/major-themes/