Final answer:
Harold Shipman, a general practitioner, was responsible for the murders of 218 patients, making him one of history's most prolific serial killers.
Step-by-step explanation:
The devious doctor responsible for 218 murders through lethal injections of diamorphine was Harold Shipman, a British general practitioner and one of the most prolific known serial killers in history. Although convicted of 15 murders, it is believed that the total number of his victims is much higher. The mention of diamorphine, an opioid medication, points to medical practice and prescribes the discipline association with medicine.