There is no doubt that both Milgram and Zimbardo caused great distress to volunteers in their studies of obedience and conformity to social roles.
The two studies show us, very dramatically, the power of the situation on human behaviour.
Milgram and Zimbardo chose ordinary people, of a sound psychological profile, not sadists and put them into challenging situations. In spite of their distress, or that of others, many volunteers continued in their violent behaviour.
Volunteers were deceived and offered money to take part, In itself this is not unusual, deception of some sort is a feature of almost all psychological experiments, and small payments encourage people to take part.
It may be argued that the volunteers were not reminded of their right to withdraw from the experiments at any time, in fact, they were encouraged to keep going, in order to see what happened.
It is important to remember that neither Milgram nor Zimbardo expected their experiments to have such dramatic effects, although this does not absolve responsibility!
To this end, Milgram consulted psychiatrists before carrying out his experiments - he debriefed and followed people up - even after a year - to make sure they weren't hurt by the experience. Most volunteers said they were pleased that they took part.
Before his prison experiment, Zimbardo used personality tests on volunteers to select stable characters.
Milgram and Zimbardo were very mindful of the state of mind of their volunteers and followed them all up carefully afterwards. Their work has had important implications for the way in which we view cases of blind obedience in real life, for example, the Jim Jones cult suicides and the running of prison systems.