Zimbardo's girl friend (later wife) shamed him into stopping the experiment which had become all too real.
Zimbardo seems awfully defensive of his famous bit of psych theater, even though a good part of his claim to fame is that he stopped it. He still needs to score off one of the "prisoners" who concluded the whole thing must have been a bit of government research on how to control antiwar protesters since Zimbardo was funded by the Office of Naval Research of the Defense Department. The suggestion still riles him.
Seems to me the gent protests too much: according to his bio as a member of the board of the American Psychological Foundation,
Zimbardo is still on the Navy's dime apparently. And probably still oblivious to -- if not willfully self-deluded about -- the fact that his intentions have little to do with how others may may use his insights into breaking down human beings.