What is the problem with conducting the forbidden experiment?
I assume that you are asking about the experiment in which a researcher would deliberately isolate a child from any contact with humans and then see how or if its language abilities develop.
What is wrong with this experiment is that it completely disrupts, and possibly destroys, the child's ability to live a normal life when it is grown up. It treats the child like an experimental animal and not like a human being. When researchers work with human subjects (at least in the United States) they must get their experiments approved to ensure that they are not doing anything that will harm the subjects physically or mentally. This experiment would clearly harm the subject emotionally in ways that would be quite profound.