Knowing that johhny wants revenge against the man who raped his sister, for example or perhaps he plans to use this knowledge to rob or hurt people himself. If we have prior knowledge do we teach and if we don't have prior knowledge, do we stop when we learn of these things or try to prevent them in the first place?
As responsible people, I assume we'd call police if we had evidence of violent crimes or threats to commit them. IMHO, we should also avoid training people (providing the tools) to accomplish those plans. If a student in this situation was a longtime member of a dojo, I assume an instructor would wish to discuss with the student why s/he was being excluded. Would this hold in other readers' dojos?