Primum non nocere. First, do no harm. This rule is a major tenant of the medical world, part of the ethics of being a doctor and a central concept of the Hippocratic oath.
Peace Corps Volunteers also take an oath. However, doing no harm plays no part in their oath.
Peace Corps does harm. International diplomacy is not a harm free activity and when reduced to the interpersonal human level this is even more true. Every year Americans are sent out to communities in seventy or so different countries around the world. Almost all of them will have life changing experiences. Some of them will be bad experiences. Some will return to America after their service, with less goodwill and friendship then they set out with.
The same is true for their host country contacts. For every hundred people who are awed and whose lives are touched by a kind American taking time out of their lives to help a poor community in a foreign land, there is one person who is angered. This person who, though at first ambivalent about America, now having met an American, bears enmity toward America and Americans.
So it is a question worth asking, does the Peace Corps do more harm then good. The answer is almost certainly worth yes. Though the noble mission of world peace and friendship is sometimes injured in the messy practicality of individuals and cultures and languages interacting, it remains nonetheless, a noble mission.