Zimbardo (who seems to enjoy his resemblance to Lucifer) conducted the Stanford Prison Experiement in 1971 in which he took average college students and turned them into either guards or prisoners. The two week experiment was ended after six days, by which time the "guards" became sadistic and the "prisoners" became depressed and showed signs of extreme stress. He also discussed several other famous experiments, in particular the Milgram Experiment, in which subjects were encouraged to "help" a student learn by administering progressively more dangerous levels of electric shock. When they resisted, they were instructed by an authority figure to continue, and 91% of the test subjects went all the way to the top of the dial--to lethal levels.
Zimbardo in his lecture, and presumably in his book, uses all of these cases to suggest that we cannot blame the "bad apple" but must rather look at the "bad barrel," or the institutional structures that create such behavior. Thus he places responsibility for the torturing of captives at Abu Ghraib upon the structure of the military itself, and not on the individuals who were corrupted under duress. His point is that any one of us would/might have done the same thing, under the circumstances.
From Zimbardo's website, The Lucifer Effect:
The “Lucifer Effect” describes the point in time when an ordinary, normal person first crosses the boundary between good and evil to engage in an evil action. It represents a transformation of human character that is significant in its consequences. Such transformations are more likely to occur in novel settings, in “total situations,” where social situational forces are sufficiently powerful to overwhelm, or set aside temporally, personal attributes of morality, compassion, or sense of justice and fair play.
Evil is the exercise of power to intentionally harm (psychologically), hurt (physically), or destroy (mortally or spiritually) others.