I found the options for your question and the correct answer would be: Zimbardo does not justify their actions, only explains how these abuses likely developed under certain conditions.
Zimbardo who is actually a highly respected psychology professor at Stanford University does not justify the action of soldiers at the prison but gives us the explanation for their behavior. He argues that the certain social conditions and the relationships between the soldiers contributed to their behavior. The influence of those more authoritative soldiers most likely played a major role in addition to the fear, boredom, stress and exhaustion along with no accountability and supervision as well as the lack of training.
