According to dave eshelman, how did researchers essentially confine the guards abusive behavior?

In The Stanford Prison Experiment, there is a lack of feedback from the researchers even though there is a dispute happening between the "prisoners" and "guards". Thus, the guards interpreted it that everything is well, that what they are doing to the prisoners, even if it is already violating and inhumane, is okay. Dave Eshelman, one of the "guards", consciously created his guard persona because he knew that the experiment has something to prove about prisons being a cruel and inhumane place.  Therefore, he did his part to help the results out of the experiment. And since no one from the researchers told him that his actions are not right, he continued acting out his character as intimidating, cold, and cruel guard.