The guards in the experiment still had the choice to NOT be cruel and they were not only cruel but violent. With encouragement but still. They had a choice. That's the important piece.
That part is very much real.

For the record the experiment was not a "fraud". It was flawed and unethical.
Yes, Zimbardo encouraged "guards" to be cruel but that doesn't mean it was "fraudulent" that means the experiment lost any validity (how accurately an experiment measures what it intends to measure).