Philip Zimbardo explains his perspective on what makes ordinary people behave in evil ways. That perspective is to give priority to situational pressures more than to personal traits. He's right to an extent: studies by others have pointed in the same direction. Christopher Browning's excellent book on Police Battalion 101 suggests that it was a desire to conform, not to let down their fellow police reservists, that played a key role in the willingness of most of the men whose testimony he studied to shoot down unarmed, helpless and innocent people in wartime Poland.
But Zimbardo overstates his case. Though he himself points out that 'there are always some who refuse and resist', who do not succumb to the pressure to conform, he at once goes on to propose 'a situational perspective for heroism'. People who bravely stand out and do something special don't necessarily do so because of their personal characteristics: it may be that 'they are more sensitive to these situational pressures' and so have mental strategies of resistance against negative social influences; in any case, we must foster an heroic imagination that would enable individuals to strengthen themselves against the noxious social forces that might one day tempt them into dehumanizing and aggressive conduct.
But this is to say, then, that some do have the inner moral strength, the moral compass, the fidelity to some basic standards of right and wrong, to withstand the type of situational pressures Zimbardo's work has been concerned with, and that he thinks that moral education, loosely speaking, can be effective. Which means that it is not only the situation that matters, it is also the nature of the actors, their courage, their moral character, their individual strength of mind.
The point here is not very profound; it is obvious in fact. But Zimbardo's exaggeration of the situational perspective undermines his own practical message. If you tell people too much that they are creatures of the situations they find themselves in, you draw attention away from the contribution they can make as individuals to changing the balance of influences that define the situation in the first place.