This is a great question, and I often raise a similar point when teaching hypnosis to students. I believe it's just a comforting myth that you can't get a person to cause harm (or rob a bank, etc.) using hypnosis, for exactly the reason you describe. If hypnosis can cause belief change, hypnosis can cause delusions, and delusions can cause people to act in harmful ways. I believe that, as hypnotherapists, we need to be vigilant of the risks in order to perform hypnosis safely and ethically.