After a relatively uneventful first day, on the second day the prisoners in Cell 1 blockaded their cell door with their beds and took off their stocking caps, refusing to come out or follow the guards' instructions. Guards from other shifts volunteered to work extra hours in order to assist in subduing the revolt, and subsequently attacked the prisoners with fire extinguishers without being supervised by the research staff. Finding that handling nine cell mates with only three guards per shift was challenging, one of the guards suggested that they use psychological tactics to control them.
etc etc
That's not normal people being coerced into doing bad things; that's "normal people" volunteering to act beyond the bounds of their instructions, and willingly acting in a manner that is outside the bounds of what is normally considered ethical or moral behavior.
And for the developer of the experiment himself:
Zimbardo aborted the experiment early when Christina Maslach, a graduate student he was then dating (and later married), objected to the conditions of the prison after she was introduced to the experiment to conduct interviews. Zimbardo noted that, of more than fifty people who had observed the experiment, Maslach was the only one who questioned its morality. After only six days of a planned two weeks' duration, the Stanford Prison experiment was discontinued
That's the guy who designed an experiment ostensibly intended to provide some insight into whether (in the words of an above commenter) Nazi guards were personally accountable for the atrocities they committed, not having any moral qualms at all about creating conditions that lead to exactly the same sort of brutality, and only stopping because his girlfriend gave him an ultimatum. Again, what we find over and over in experiments like this, is that people will often willingly volunteer to escalate to acts of violence, if given official sanction to do so (and decidedly not, as you claim, and I wish, because they were manipulated or coerced).
And like I said from the beginning, read up on the experience of the dictatorial regimes of the last century if you need more evidence. You don't have to force people, or even prod them or train them, to come up with and commit acts of horrific cruelty. You just have to provide the venue for it, and the people will show up of their own accord.
New York... when civilization falls apart, remember, we were way ahead of you. - David Letterman