One key theme of The Lucifer Effect addresses the potential of all humans to behave in evil ways if certain situational and environmental factors are in place. Zimbardo describes in detail the processes of the Stanford Prison Experiment, a psychology research experiment that revealed the susceptibility of the participants to behave according to the expectations of their roles rather than according to their own moral compasses. The experience of ex-con Carlo Prescott is particularly illustrative of this susceptibility; Prescott, a man long accustomed to the experiences of a prisoner, quickly adapted to his role as chief parole officer within the context of the prison experiment. Though he himself knew well how it felt to be treated unjustly and to be denied parole, Prescott behaved in an authoritarian manner, acknowledging later that the power of his role had undermined his empathy for the prisoners.

To illustrate his belief that all humans are capable of evil, Zimbardo applies the findings that emerged from the Stanford Prison Experiment to instances of torture and abuse. His analysis of the Abu Ghraib prison abuse scandal suggests that the individual interrogators and military personnel who were found guilty of abuse committed the acts of torture under extreme circumstances.