...except that Doc was well versed in the use of ether to treat dental patients; in fact, that's one of the reasons he went to dental school and not medical school. His uncle, whom he was named after, was a doctor, and counseled him that the most advanced training at the time was in dentistry, not medicine, precisely because the medical profession had not yet adopted the widespread use of ether. Doctors thought pain was morally useful, and refused to use it in, for example, childbirth, reasoning that God intended women to bear children in pain. But for some reason (possibly because men suffered from dental problems but not childbirth pains), it was okay to use ether in dentistry. So John Henry hied himself to Pennsylvania to study tooth doctoring, because it was technically more advanced than medicine. Given that history, it's likely that Doc used ether on his patients, and thus was not inflicting as much pain as we think.

As for his cough, it was annoying but not necessarily frightening; in Doc's time, tuberculosis was still considered to be hereditary, not contagious. It was not identified as an infectious disease until 1882, and I doubt the news had reached Tombstone until well afterwards.

It may be that Doc was violent because he was raised in a violent environment; the same could be said of every male his age. The slums of New York, the prairies of Indiana, and the lawless Gold Rush era of California certainly produced some hair-trigger types, quick to take offense, and well lubricated by liquor. Add to that the fact that, as a Southerner, he had lingering resentments against the Union that defeated the South (as did the Clantons). I think Doc would have found himself as violence-prone in Alabama or Kentucky as in Arizona.