That doesn't seem to be the conclusion of said experiment.
"In Milgram's first set of experiments, 65 percent (26 of 40)[1] of experiment participants administered the experiment's final massive 450-volt shock, though many were very uncomfortable doing so; at some point, every participant paused and questioned the experiment, some said they would refund the money they were paid for participating in the experiment."
IMHO Milgram's experiment has more to do with obedience than ethics.
Or they felt trapped by their agreement to participate in the experiment. They made an offer to annul the contract, which would indicate that they were following through with what they had agreed to do. Moreover, the situation they were in was greatly outside their experience with what people had ever asked them to do in fulfilling an agreement.
If you promised someone "I'll do whatever it takes for you to have a good time while you're visiting", you probably aren't imagining that it will involve a triple homicide and a liquor store robbery. If it starts there, you'll likely refuse. But if you slowly build up to that, the pressure to keep complying with your agreement builds....
I am familiar with the psychological phenomenon you are referring to, but these people were not forced to do anything; they clearly considered it wrong; and yet they proceeded. Feeling trapped isn't being trapped. Being rude to a researcher is not as bad as physically injuring an innocent person. Excuses do not change the fundamental reality that people commonly do things they know to be wrong and suboptimal – we are so far from perfect agents! Improving this through rationality is the whole purpose of e.g. lesswrong.
You mention LessWrong, very nice; then you know they were forced: any time you deploy the Dark Arts to manipulate someone's behavior, you undermine their free will. "Choice" wasn't part of it, or at least not the largest part.
