Anonymity in Stanford Prison Experiment

The guards in the Stanford Prison Experiment did things that got worse and worse as the experiment went on, but I cannot determine if the cause for all of these terrible things is mainly due to the fact that they could remain completely anonymous when doing these crimes by wearing their sunglasses, or just because what they were doing they perceived as being right, because they had no reference point for how prison guards were supposed to act. I think it is mainly due to the fact that this is an isolated experiment, and the guards thought they were doing the right thing, much like the Japanese in the Rape of Nanking. Thoughts?