Zimbardo does not justify their actions, only explains how these abuses likely
developed under certain conditions.
In the article, Zimbardo refers to his own experiments about human behavior in certain conditions to explain the reasons behind the soldiers' behavior. He states that their mental state allowed for their actions because of "stress, fear, boredom and heat exhaustion, coupled with no supervision, no training, and no accountability." In the text he never states that their behavior is acceptable or justified given the conditions, he is merely explaining how good people can do bad things.
