Golding wrote Lord of the Flies shortly after learning of the horrors of World War II and the Holocaust. One question facing many people at the time was: "How can 'good' people commit such horrific crimes?" What makes good people do bad things?
At the beginning of the unit, I take one student aside and let them know I have candy in my desk. I announce to the class the need to "run to the office" or "go into my backroom for a few minutes." As soon as I am out of sight, my accomplice gets the candy out and offers it to his classmates: "Come one, the teacher will never know."
Some students accept the candy, others do not. I return and make them aware of the ruse.
We end up having a very good discussion about peer pressure and "why good people do bad things."