Zimbardo's Study

Hell all,
I am taking a Psychology class, and we are just studying personality. The professor showed us a video about the experiment that Dr. Zimbardo led back in 1971. Dr. Zimbardo was trying to prove how good people can become evil when put in an evil place. It was very interesting because what started off as a simple experiment, became something that got out of hand. The participants that volunteered for this experiment were assigned to be either the guard or an inmate by a coin toss. A building in Stanford University was made to look like a prison with the bar cells and all. In a matter of a day, the inmates started to believe in the crimes that they supposedly got arrested for. The guards really got in the part of the authority and one of them even tortured the inmates. The inmates even refered to themselves by their inmate number not their name. They all rebeled against the guards just like in the real prisons, but things got out of hand. The students were playing the part so well, one of the inmates had to get psychiatric help after the experiment. The experiment was scheduled to last 2 weeks but was stopped in 6 days instead.
My question is: Can good people become change when put in an evil place? I'd personally like to think that this is not the case. But what do you think? Do you believe we react to what is called a situational attribution. Situational attribution meaning, we act depending in the situation we are assigned or exposed to.
For more information about this study, I have provided the website http://www.prisonexp.org/.