There are issues of foreseeability, lack of privity, and as always in malpractice cases, Did the prescribing of the medication deviate from accepted practice and did it proximately cause the events in question. Need more details to help.
Not clear what your involvement in all this is as either patient, victim or concerned citizen. Regardless, I am sorry to hear about this. You can contact (in writing) Medical Provider's Administration (if there is one) and report the incident to internal investigation and review. Or you can contact the State Department of Health to report the incident. If you decide to communicate your concerns to treating doctor, do so in writing, now rather than late. Document everything in writing to create paper trail to support claim(s) on your behalf or others. Medical malpractice is professional negligence: 1) caused by health care provider's act or omission where treatment provided falls below accepted standard of practice in the medical community; AND 2) causes injury or death to the patient, with most cases involving medical error. Best to consult a local attorney in your jurisdiction with the specifics of matter. Click on "Find A Lawyer" tab above and you will be able to figure out how to best move forward
If the doctor knew or should have known that the person would have the kind of reaction that would cause him to harm others, you can argue that it was foreseeable. However, if the person that ended up committing the killing has never had such a reaction before and the doctor had no way of knowing about the murderous propensity, then you will face an uphill battle. You would need to get the murderer's medical records to see if the doctor was on notice.