The easiest way to cover up deliberate harm to a patient is to use a psychiatric disorder. How can trust exist?

Trust is important. Physician patient relationship is dependent on trust. All physicians strive to do just that. Unfortunate things do happen and at times result in harm even with care. Psychiatric disorders are legitimate medical disorders and cannot be diagnosed at whim of a disgruntled physician as there are definite criteria for diagnosis. If one is unsatisfied a second opinion can be sought about diagnosis.
Trust goes 2 ways. A psychiatric disorder is diagnosed when a person suffers symptoms and history that meet research-based criteria for that disorder. The purpose is to understand and treat the person so that s/he moves towards health and less suffering. Mutual doctor-patient trust is needed. You suggest here that doctors intend to harm, not help, others. Consulting with a doctor you're willing to trust, may help.