In reality, a very large dose of morphine may well cause death if given to a healthy person who is not in pain and has not received morphine before. But when administered for pain, such drugs are taken up first by the patient's pain receptors. In fact, patients regularly receiving morphine for pain quickly build up a resistance to side-effects such as respiratory suppression, so they can easily tolerate doses that would cause death in other people. Fortunately they build up a tolerance to the side-effects far more quickly than to the drugs' analgesic effects so doctors need not hesitate to increase dosages when needed to relieve pain. As long as a patient is awake and in pain, the risk of hastening death by increasing the dose of narcotics is virtually zero.
Unrelieved pain is itself a stimulant, which overwhelms any depressive effects of narcotics.
Patients whose unrelieved pain is distorting the very fabric of their lives need adequate pain control the way a diabetic needs insulin to function properly. Very rarely it may be necessary to induce sleep to relieve pain and other distress in the final stage of dying. While some terminally ill patients may die under such sedation, this is generally because they were imminently dying already. It does not kill the patient, but it can make his or her suffering bearable. The factual evidence supports these claims.